Biomedical subjects

D A Low

Publications and source records attributed to D A Low.

At least 73 records · Page 4Linked to original sources

PubMed1995

A convolution-adapted ratio-TAR algorithm for 3D photon beam treatment planning.

A convolution-adapted ratio of tissue-air ratios (CARTAR) method of dose calculation has been developed at the Mallinckrodt Institute of Radiology. This photon pencil-beam algorithm has been developed and implemented specifically for three-dimensional treatment planning. In a standard ratio of tissue-air ratios (RTAR) algorithm, doses to points in irregular field geometries are not adequately modeled. This is inconsistent with the advent of conformal therapy, the goal of which is to conform the dose distribution to the target volume while sparing neighboring sensitive normal critical structures. This motivated us to develop an algorithm that can model the beam penumbra near irregular field edges, while retaining much of the speed for the original RTAR algorithm. The dose calculation algorithm uses two-dimensional (2D) convolutions, computed by 2D fast Fourier transform, of pencil-beam kernels with a beam transmission array to calculate 2D off-axis profiles at a series of depths. These profiles are used to replace the product of the transmission function and measured square-field boundary factors used in the standard RTAR calculation. The 2D pencil-beam kernels were derived from measured data for each modality using commonly available dosimetry equipment. The CARTAR algorithm is capable of modeling the penumbra near block edges as well as the loss of primary and scattered beam in partially blocked regions. This paper describes the dose calculation algorithm, implementation, and verification.

Algorithms↗

PubMed1995

Measurement of a photon penumbra-generating kernel for a convolution-adapted ratio-TAR algorithm for 3D treatment planning.

A method has been developed to measure a photon penumbra-generating kernel using dosimetry equipment available in most radiation therapy departments. The kernel is used in a convolution-adapted ratio-TAR algorithm in our three-dimensional treatment planning system. The kernel is assumed to be invariant with respect to off-axis position, axially symmetric, and is divided into short- and long-range components, with a different measurement technique for each. The data required to obtain the short-range component are measured by scanning across a split-field geometry incident on a water phantom. The derivative of the measured profile is proportional to one-dimensional projections across the kernel. Because the kernel is axially symmetric, only one profile measurement is required for each depth. A CT reconstruction technique is used to extract the radial dependence of the kernel from the strip integrals. Electronic noise in the acquisition system yields significant uncertainties in the kernel shape for distances beyond 3 cm. The long-range portion of the kernel is obtained by examining tissue-air ratios (TARs). The derivative of the TAR at the center of a circular field is proportional to the kernel value at the distance corresponding to the radius of the field. The kernel measurement method was tested by comparing measured and calculated square-field profiles at a variety of depths. Agreement was within 1% within the field boundary and 3% outside the field boundary for all depths.

Algorithms↗

PubMed1995

The evaluation of optimized implants for idealized implant geometries.

The purpose of this paper is to investigate the utility of implant quality measures on single stepping-source brachytherapy treatment plans. Four dwell weight optimization algorithms were applied to four regular geometric implants: single plane, double plane, cuboid, and cylindrical. The dwell weight optimization schemes included equal weighing, two commercial optimization schemes (dose-point and geometric) and a variation of the Paterson-Parker distribution rules. The implant quality measures were investigated as a function of dose-per-integrated reference air kerma (IRAK) to eliminate bias resulting from a prescription choice. A particular dose per IRAK refers to a dose surface that is a function only of the relative dwell weight distribution and is therefore well suited to investigate dwell weight optimization schemes. The implant quality measures included the dose-nonuniformity ratio (DNR) developed by Saw and a coverage index to assess the isodose coverage relative to the implanted volume. These were termed direct quantities due to their clear clinical significance. Additional measures include the ratio of the implant dose-volume histogram (DVH) to that of a point source exhibiting the same IRAK (Rp) and the ratio of the optimized DVH to the equally weighted DVH (EWR). The widths of the Rp curves and depths of the EWR curves were used to characterize these indirect implant quality measures. To evaluate the effectiveness of both the direct and indirect measures, they were correlated with the DNR for an isodose surface that covered the implant (D0). The efficiency of the dwell weight distribution was examined by noting the dose-per-IRAK surface D0.(ABSTRACT TRUNCATED AT 400 WORDS)

Algorithms↗

PubMed1995

Specific binding of PapI to Lrp-pap DNA complexes.

Expression of pyelonephritis-associated pili (Pap) varies between transcriptionally active (ON) and inactive (OFF) phase states. Pap phase variation is controlled by the binding of leucine-responsive regulatory protein (Lrp) to two pap regulatory DNA regions, each containing a deoxyadenosine methylase site and designated GATC-I and GATC-II. Methylation of these GATC sites modulates binding of Lrp and plays an essential role in phase variation. PapI, an 8.8-kDa pap-encoded regulatory protein, plays a key role in the switch between OFF and ON transcription states. In the absence of PapI, Lrp binds to sites overlapping the papBA promoter and inhibits transcription. Addition of PapI results in a translocation of Lrp binding to sites over 100 bp upstream, resulting in the ON transcription state. Gel shift analysis using radiolabeled PapI shows that PapI binds with high specificity to Lrp-pap DNA complexes but binds only weakly to free Lrp. Protein cross-linking studies indicate that Lrp and PapI directly interact with each other. On the basis of these data, we present a hypothesis in which PapI facilitates the transition between OFF and ON transcription states by binding to Lrp and altering Lrp's affinity for the pap GATC-I and GATC-II regions.

Bacterial Proteins↗

PubMed1994

Methylation patterns in pap regulatory DNA control pyelonephritis-associated pili phase variation in E. coli.

We have examined the roles of pap DNA methylation patterns in the regulation of the switch between phase ON and OFF pyelonephritis-associated pili (Pap) expression states in E. coli. Two Dam methyltransferase sites, GATC1028 and GATC1130, were shown previously to be differentially methylated in phase ON versus phase OFF cells. In work presented here, these sites were mutated so that they could not be methylated, and the effects of these mutations on Pap phase variation were examined. Our results show that methylation of GATC1028 blocks formation of the ON state by inhibiting the binding of Lrp and PapI regulatory proteins to this site. Conversely, methylation of GATC1130 is required for the ON state. Evidence indicates that this occurs by the inhibition of binding of Lrp to sites overlapping the pilin promoter. A model describing how the transition between the phase ON and OFF methylation states might occur is presented.

Bacterial Proteins↗

PubMed1994

Determination of the relative linear collision stopping power and linear scattering power of electron bolus material.

The linear collision stopping power and linear scattering power for machineable wax relative to water have been determined for electron energies between 2 and 20 MeV. Knowledge of these quantities is necessary for the use of this wax as bolus in electron pencil-beam dose algorithms. The atomic composition of the wax (rho = 0.920 +/- 0.001 g cm(-3)) was obtained by having the wax assayed. The formalisms expressed in the ICRU Report 35 were used to calculate the relative linear collision stopping and linear scattering powers of the wax. The calculated relative linear collision stopping powers of 2 to 20 MeV electrons in the wax ranged from 0.949 +/- 0.005 to 0.952 +/- 0.005, and the calculated relative linear scattering powers ranged from 0.734 +/- 0.004 to 0.729 +/- 0.004. As a check of the calculation method, the relative linear collision stopping power was measured by determining the shift in electron central-axis depth-ionization curves when varying thicknesses of water were replaced by wax. These measurements, made using 10, 12, 15 and 18 MeV electron beams with wax thicknesses from 1.0 - 4.0 cm, resulted in a mean value of 0.931 +/- 0.008. Determination of the relative linear stopping power and the linear scattering power by using the measured CT number to extract values from patient data tables resulted in values of 0.933 +/- 0.009 and 0.746 +/- 0.016, respectively, indicating that it should be acceptable to use the Hounsfield values obtained with CT scans for treatment planning dose calculations.

Algorithms↗

PubMed1994

Leucine-responsive regulatory protein and deoxyadenosine methylase control the phase variation and expression of the sfa and daa pili operons in Escherichia coli.

The Escherichia coli operons daa and sfa encode F1845 and S pili, respectively. In this paper we show that the expression of these operons is under phase variation control at a transcriptional level. The transcription of both operons is dependent on the global regulator leucine-responsive regulatory protein (Lrp) and deoxyadenosine methylase (Dam). Lrp is required for methylation protection of two GATC sites located within conserved DNA sequences in the regulatory regions of these operons. These GATC sites are differentially methylated, establishing a methylation pattern which is characteristic of either the phase ON or phase OFF state. We also show that Lrp binds to the daa and sfa regulatory regions and that this binding is modulated by the methylation of the GATC sites. These results indicate that the phase variation of the daa and sfa operons is regulated by a mechanism involving differential binding of Lrp owing to methylation of GATC sites in the regulatory region, which is similar to the mechanism that controls phase variation of the pap operon.

Adhesins, Escherichia coli↗

PubMed1994

Analysis of nonmethylated GATC sites in the Escherichia coli chromosome and identification of sites that are differentially methylated in response to environmental stimuli.

Seven GATC sites that are nonmethylated in logarithmic growth phase cells using glycerol as a carbon source were isolated from the Escherichia coli chromosome. Three of these GATC sites are located upstream of the operons gut, mtl, and ppiA, whereas DNA sequences adjacent to three other nonmethylated GATC sites are not homologous to previously identified genes. The seventh nonmethylated GATC site is located downstream of uspA. The protection of this site from DNA methylation requires leucine-responsive regulatory protein and is leucine responsive. The carbon source and the growth phase influenced the protection of the GATC site 5' of the ppiA gene. The other five sites were protected under all the environmental conditions examined.

Bacterial Proteins↗

PubMed1993

Iodine replacement in fibrocystic disease of the breast.

OBJECTIVE: To determine the response of patients with fibrocystic breast disease to iodine replacement therapy. DESIGN: Review of three clinical studies beginning in 1975: an uncontrolled study with sodium iodide and protein-bound iodide; a prospective, control, crossover study from iodide to molecular iodine; and a prospective, control, double-blind study with molecular iodine. SETTING: University affiliated breast-treatment clinics. PATIENTS: Study 1: 233 volunteers received sodium iodide for 2 years and 588 received protein-bound iodide for 5 years. Study 2: the treatment of 145 patients from study 1 treated with protein-bound iodide for several months who still had symptoms was switched to molecular iodine 0.08 mg/kg; 108 volunteers were treated initially with molecular iodine. Study 3: 23 patients received molecular iodine, 0.07 to 0.09 mg/kg body weight; 33 received an aqueous mixture of brown vegetable dye and quinine. The numbers in study 2 increased over the review period so that 1365 volunteers were being treated with molecular iodine by 1989. INTERVENTIONS: All patients in study 3 had pre- and post-treatment mammography and measurement of serum triiodothyronine, thyroxine and thyroid-stimulating hormone levels. MAIN OUTCOME MEASURES: Subjective evaluation--freedom from pain--and objective evaluation--resolution of fibrosis. RESULTS: Study 1: 70% of subjects treated with sodium iodide had clinical improvement in their breast disease, but the rate of side effects was high; 40% of patients treated with protein-bound iodide had clinical improvement. Study 2: 74% of patients in the crossover series had clinical improvement, and objective improvement was noted in 72% of those who received molecular iodine initially. Study 3: in the treatment group 65% had subjective and objective improvement; in the control group there was a subjective placebo effect in 33% and an objective deterioration of 3%. CONCLUSIONS: The fibrocystic breast reacts differently to sodium iodide, protein-bound iodide and molecular iodine. Molecular iodine is nonthyrotropic and was the most beneficial.

Adolescent↗

PubMed1992

Leucine-responsive regulatory protein controls the expression of both the pap and fan pili operons in Escherichia coli.

The methylation blocking factor gene (mbf) in Escherichia coli is required for specific methylation inhibition of two DNA GATC sites upstream of the papBA pilin promoter and transcriptional activation of pap. Complementation and mutational analysis using pap-lac and ilvIH-lac operon fusions indicates that the mbf gene is identical to a recently described global regulatory gene lrp (leucine-responsive regulatory protein) that acts as a positive regulator of some genes and a negative regulator of others in E. coli. DNA sequence analysis of an mbf::mTn10 insertion showed that the mbfDNA sequence was identical to lrp. Thus Lrp inhibits DNA methylation at specific GATC sites. We also show that Lrp positively regulates transcription of the fan operon, which encodes K99 pili of diarrheagenic E. coli. Purified Lrp was found to bind to DNA fragments encompassing the pap and fan promoters, which is consistent with previous results indicating that Lrp controls gene expression by binding to regulatory DNA sites. Exogenous leucine significantly reduced fan transcription and K99 pili expression, similar to results obtained with the ilvIH operon. However, pap gene expression was unresponsive to leucine, which distinguishes pap from other lrp-regulated genes whose expression is modulated by leucine.

Amino Acid Sequence↗

PubMed1992

Evidence for global regulatory control of pilus expression in Escherichia coli by Lrp and DNA methylation: model building based on analysis of pap.

Pyelonephritis-associated pilus (Pap) expression is regulated by a phase variation control mechanism involving PapB, Papl, catabolite activator protein (CAP), leucine-responsive regulatory protein (Lrp) and deoxyadenosine methylase (Dam). Lrp and Papl bind to a specific non-methylated pap regulatory DNA region containing the sequence 'GATC' and facilitate the formation of an active transcriptional complex. Evidence indicates that binding of Lrp and Papl to this region inhibits methylation of the GATC site by Dam. However, if this GATC site is first methylated by Dam, binding of Lrp and Papl is inhibited. These events lead to the formation of two different pap methylation states characteristic of active (ON) and inactive (OFF) pap transcription states. The fae (K88), daa (F1845) and sfa (S) pilus operons share conserved 'GATC-box' domains with pap and may be subject to a similar regulatory control mechanism involving Lrp and DNA methylation.

Bacterial Proteins↗

PubMed1992

Thermoregulation of the pap operon: evidence for the involvement of RimJ, the N-terminal acetylase of ribosomal protein S5.

Our previous work showed that pap pilin gene transcription is subject to a thermoregulatory control mechanism under which pap pilin is not transcribed at a low temperature (23 degrees C) (L. B. Blyn, B. A. Braaten, C. A. White-Ziegler, D. H. Rolfson, and D. A. Low, EMBO J. 8:613-620, 1989). In order to isolate genes involved in this temperature regulation of gene expression, chromosomal mini-Tn10 (mTn10) mutations that allowed transcription of the pap pilin gene at 23 degrees C were identified, and the locus was designated tcp, for "thermoregulatory control of pap" (C. A. White-Ziegler, L. B. Blyn, B. A. Braaten, and D. A. Low, J. Bacteriol. 172:1775-1782, 1990). In the present study, quantitative analysis showed that the tcp mutations restore pap pilin transcription at 23 degrees C to levels similar to those measured at 37 degrees C. By in vivo recombination, the tcp mutations were mapped to phage E4H10S of the Kohara library of the Escherichia coli chromosome (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987). The tcp locus was cloned by complementation, in which a 1.3-kb DNA fragment, derived from the Kohara phage, was shown to restore thermoregulation to the mTn10 mutants. DNA sequencing revealed two open reading frames (ORFs) encoding proteins with calculated molecular masses of 22.7 and 20.3 kDa. The sequence of the 22.7-kDa ORF was identical to that of rimJ, the N-terminal acetylase of the ribosomal protein S5. The gene encoding the 20.3-kDa ORF, designated g20.3 here, did not display significant homology to any known DNA or protein sequence. On the basis of Northern (RNA) blot data, rimJ and g20.3 are located within the same operon. Two of the mTn10 transposons in the thermoregulatory mutants were inserted within the coding region of rimJ, indicating that the RimJ protein plays an important role in the temperature regulation of pap pilin gene transcription. However, rimJ itself is not thermoregulated, since rimJ transcripts were detected at both 23 and 37 degrees C. Disruption of the g20.3 gene by insertion and deletion mutagenesis did not affect thermoregulation of the pap operon, suggesting that, although g20.3 lies within the same operon as rimJ, it does not play a role in thermoregulation.

Acetylation↗

PubMed1991

Effect of dimensionality of heterogeneity corrections on the implementation of a three-dimensional electron pencil-beam algorithm.

Electron beam dose distributions were calculated on a three-dimensional grid using three pencil-beam algorithms, each taking into account irregularities in field shape. The algorithms differ in that patient anatomy in either one, two, or three dimensions is used in the calculation of dose to a point. Algorithms were optimized for speed by such techniques as precalculation and storage of several quantities, reordering of pencil-beam and grid-point loops, selection of cut-off values for some calculated quantities, and invoking error function symmetries. Execution times for optimized versions of each of the algorithms as implemented on a three-dimensional treatment planning system were comparable for both the one- and two-dimensional heterogeneity correction requires an additional calculational loop over fan lines. Execution times for the three-dimensional heterogeneity correction were approximately a factor of four longer than those for the two-dimensional correction. For certain geometries, three-dimensional heterogeneity corrections were necessary to calculate dose distributions accurately, in spite of the additional cost in calculation times.

Algorithms↗

PubMed1991

Legionella pneumophila inhibits protein synthesis in Chinese hamster ovary cells.

Legionella pneumophila is a gram-negative facultative intracellular parasite that causes Legionnaires disease. To explore the interactions between L. pneumophila and host cells, we have developed a continuous cell line model of infection. We show that about 80% of Chinese hamster ovary (CHO) cells were associated with L. pneumophila after incubation for 3 h at a multiplicity of infection of 20 bacteria per cell. Within 3 to 4 h of incubation with L. pneumophila, protein synthesis of CHO cells was markedly inhibited, as shown by the reduction of incorporation of radiolabeled amino acids into proteins. L. pneumophila did not inhibit transport of amino acids or cause degradation of newly synthesized proteins in CHO cells. Cytochalasin D blocked internalization of L. pneumophila by CHO cells, yet CHO cell protein synthesis was inhibited. These results indicated that L. pneumophila could inhibit host protein synthesis from the cell exterior. L. pneumophila that had been killed with antibiotics prior to incubation with CHO cells still inhibited protein synthesis, indicating that the inhibition of CHO cell protein synthesis occurred in the absence of de novo protein synthesis by L. pneumophila.

Animals↗

PubMed1991

DNA sequences of three papA genes from uropathogenic Escherichia coli strains: evidence of structural and serological conservation.

Pyelonephritis-associated pili (Pap) are important in the pathogenesis of ascending, unobstructive Escherichia coli-caused renal infections because these surface bacterial organelles mediate digalactoside-specific binding to host uroepithelial cells. Pap are composed of many different polypeptides, of which only the tip proteins mediate specific binding. The PapA moiety polymerizes to form the bulk of the pilus structure and has been employed in vaccines despite its lack of Gal alpha(1-4)Gal receptor specificity. Animal recipients of PapA pilus-based vaccines are protected against experimental pyelonephritis caused by homologous and heterologous Gal-Gal-binding uropathogenic E. coli strains. Specific PapA immunoglobulin G antibodies in urine are correlated with protection in these infection models. The nucleotide sequences of the gene encoding PapA were determined for three E. coli clones expressing F7(1), F7(2), and F9 pili and were compared with corresponding sequences for other F serotypes. Specific rabbit antisera were employed in enzyme-linked immunosorbent assays to study the cross-reactivity between Gal-Gal pili purified from recombinant strains expressing F7(1), F7(2), F9, or F13 pili and among 60 Gal-Gal-binding wild-type strains. We present data which corroborate the concept that papA genes are highly homologous and encode proteins which exhibit greater than 70% homology among pili of different serotypes. The differences primarily occur in the cysteine-cysteine loop and variable regions and constitute the basis for serological diversity of these pili. Although there are differences in primary structures among these pili, antisera raised against pili of one serotype cross-reacted frequently with many other Gal-Gal pili of different serotypes. Furthermore, antisera raised against pili of the F13 serotype cross-reacted strongly or moderately with 52 (86%) of 60 wild-type Gal-Gal-binding E. coli strains. These data suggest that there are common immunogenic domains among these proteins. These additional data further support the hypothesis that broadly cross-protective PapA pilus vaccines for the immunoprophylaxis of pyelonephritis might be developed.

Amino Acid Sequence↗

PubMed1991

Evidence for a methylation-blocking factor (mbf) locus involved in pap pilus expression and phase variation in Escherichia coli.

Transcription of the pyelonephritis-associated pilus (pap) operon of Escherichia coli is subject to regulation by a phase variation control mechanism in which the pap pilin gene alternates between transcriptionally active (phase-on) and inactive (phase-off) states. Pap phase variation appears to involve differential inhibition of deoxyadenosine methylase (Dam) methylation of two pap GATC sites, GATC1028 and GATC1130, located in the regulatory region upstream of the papBA promoter. DNA from phase-on cells contains an unmethylated adenosine in the GATC1028 site, whereas DNA from phase-off cells contains an unmethylated adenosine in the GATC1130 site. papI and papB are two regulatory genes in the pap operon. Analysis of pap deletion mutants suggests that papI is required for methylation inhibition at the GATC1028 site; however, neither papI nor papB is required for inhibition of methylation at the GATC1130 site. We have identified a chromosomal locus, mbf (methylation-blocking factor), that is required for methylation protection of both the pap GATC1028 and GATC1130 sites. The mbf locus was identified after transposon mTn10 mutagenesis and mapped to 19.6 min on the E. coli chromosome. The effect of transposon mutations within mbf on pap pilin transcription was determined by using a papBAp-lac operon fusion which places lacZ under control of the papBA promoter. E. coli containing mbf::mTn10 and phase-off mbf+ E. coli cells both expressed beta-galactosidase levels about 30-fold lower than the beta-galactosidase level measured for phase-on mbf+ E. coli cells. These results indicated that mbf was necessary for pap pilin transcription and were supported by Northern (RNA) blotting and primer extension analyses. Moreover, transposon insertion within mbf greatly reduced Pap pilus expression. The mbf locus was isolated on a low-copy-number cosmid, pMBF1. Complementation analysis indicated that each of seven mbf::mTn10 mutants isolated contained a transposon insertion within the same gene or operon. The identification of the mbf locus, required for pap transcription, supports the hypothesis that pap phase variation is controlled by a mechanism involving alternation between different methylation states.

Amino Acid Sequence↗

PubMed1990

Regulation of pap pilin phase variation by a mechanism involving differential dam methylation states.

Transcription of the pap pilin (papA) gene in Escherichia coli is subject to control by a heritable phase variation mechanism in which alternation between transcriptionally active (phase on) and inactive (phase off) states occurs. Our results suggest that phase switching occurs without DNA rearrangement of pap DNA sequences, distinguishing this system from those described for E. coli type 1 pili and Salmonella flagellar phase variation. Analysis of the regulatory region upstream of papA in DNAs isolated from phase off and phase on cell populations showed that two deoxyadenosine methylase (Dam) sites, GATC1028 and GATC1130, were present. Southern blot analysis of MboI and DpnI restriction digests of DNAs showed that the GATC1028 site was unmethylated only in DNA isolated from phase on populations. Conversely, GATC1130 sites were unmethylated in DNA isolated from phase off populations. The presence of unmethylated GATC sites in E. coli is unusual and to our knowledge has not been previously reported. These results suggest that the methylation states of GATC1028 and GATC1130 may regulate pap transcription. Consistent with this hypothesis, Dam methylase levels affected the regulation of pap transcription; papA transcription was absent in dam- E. coli. Moreover, transition from the phase off to phase on state was not observed in E. coli expressing aberrantly high levels of Dam. A basic model is presented which outlines a possible mechanism by which alternation between phase off and phase on methylation states could occur.

Amino Acid Sequence↗

PubMed1990

Identification of an Escherichia coli genetic locus involved in thermoregulation of the pap operon.

We previously showed, using a single-copy papBAp-lac fusion (previously designated papBA-lac), that pyelonephritis-associated pili (pap) pilin gene transcription is subject to both phase variation and thermoregulatory control mechanisms (L. B. Blyn, B. A. Braaten, C. A. White-Ziegler, D. H. Rolfson, and D. A. Low, EMBO J. 8:613-620, 1989). At 37 degrees C, Escherichia coli strains carrying the papBAp-lac fusion displayed both Lac+ and Lac- colony phenotypes. In contrast, at 23 degrees C, colonies displayed a uniform Lac- phenotype, suggesting that pilin was not transcribed at this temperature. In this study, a strain carrying the papBAp-lac fusion was subjected to mini-Tn10 (mTn10) mutagenesis to isolate mutants that could initiate transcription of pilin at the nonpermissive temperature. Two classes of thermoregulatory mutants were identified in which the mTn10 mutation was linked to the mutant phenotype. Class I mutants displayed a phase variation phenotype at both 37 degrees C and 23 degrees C, whereas class II mutants displayed a uniform Lac+ colony phenotype at both temperatures. Preliminary analysis of these mutants showed that the mTn10 insertions in the class I mutants were chromosomally located, whereas the mTn10 insertions in the class II mutants were located within the papBAp-lac fusion phage. Southern blot analysis of the class I mutants demonstrated that mTn10 was present in the same 5.9-kilobase SalI DNA fragment in each mutant. Two of the class I mTn10 mutations were mapped to approximately 23.4 min on the E. coli K-12 chromosome. The locus defined by the class I mTn10 mutations was designated tcp, for thermoregulatory control of pap. Analysis of phase transition rates of the class I mutants showed that the phase-off (Lac-)----phase-on (Lac+) transition rates were higher than those observed with the nonmutant E. coli strain.

Blotting, Southern↗