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Biomedical subjects

M Lieb

Publications and source records attributed to M Lieb.

At least 37 records · Page 2Linked to original sources
PubMed1988

Very short patch mismatch repair activity associated with gene dcm is not conferred by a plasmid coding for EcoRII methylase.

The only cytosine methylase in Escherichia coli K-12 methylates the second cytosine in the sequence CC (A/T)GG and is encoded by gene dcm. Methylation and very short patch mismatch repair activities lacking in a dcm mutant of E. coli were restored by a plasmid containing the cloned dcm gene. In contrast, plasmids with the gene for EcoRII methylase, which is a homolog of dcm, restored only cytosine methylase activity and not mismatch repair.

DNA (Cytosine-5-)-Methyltransferases↗
PubMed1987

Amelioration of experimental lipid keratopathy by photochemically induced thrombosis of feeder vessels.

The photochemical interaction between intravenously injected rose bengal dye and 514.5-nm argon laser irradiation was employed to initiate permanent thrombotic occlusions in the corneal neovasculature of rabbit eyes with experimentally induced lipid keratopathy. This photothrombotic procedure did not produce corneal edema or polymorphonuclear leukocyte infiltration, as was found in previous studies employing laser-induced photocoagulation. Thus, by means of the new technique, we avoided the formation of additional neovascularization and lipid deposition. Vascular occlusion by photothrombosis, effected with 8.5 times less incident intensity and 27.5 times less total light exposure than with photocoagulation, yielded an average reduction of corneal cholesterol content of 36% as opposed to an increase of 24% found with previous argon laser photocoagulation.

Animals↗
PubMed1987

Rat brain microvessel extracellular matrix modulates the phenotype of cultured rat type 1 astroglia.

Astroglia from immature rat cerebral white matter which were plated on the insoluble extracellular matrix (ECM) secreted by rat cerebral microvessel endothelial cells (RCMEC) and maintained in a defined medium were induced to become stellate and to express glutamine synthetase. These effects were not elicited by RCMEC-conditioned medium. ECM secreted by rat pleural mesothelium elicited a lesser proportion of stellate astroglia and did not induce glutamine synthetase.

Animals↗
PubMed1987

Bacterial genes mutL, mutS, and dcm participate in repair of mismatches at 5-methylcytosine sites.

Certain amber mutations in the cI gene of bacteriophage lambda appear to recombine very frequently with nearby mutations. The aberrant mutations included C-to-T transitions at the second cytosine in 5'CC(A/T)GG sequences (which are subject to methylation by bacterial cytosine methylase) and in 5'CCAG and 5'CAGG sequences. Excess cI+ recombinants arising in crosses that utilize these mutations are attributable to the correction of mismatches by a bacterial very-short-patch (VSP) mismatch repair system. In the present study I found that two genes required for methyladenine-directed (long-patch) mismatch repair, mutL and mutS, also functioned in VSP mismatch repair; mutH and mutU (uvrD) were dispensable. VSP mismatch repair was greatly reduced in a dcm Escherichia coli mutant, in which 5-methylcytosine was not methylated. However, mismatches in heteroduplexes prepared from lambda DNA lacking 5-methylcytosine were repaired in dcm+ bacteria. These results indicate that the product of gene dcm has a repair function in addition to its methylase activity.

5-Methylcytosine↗
PubMed1986

Characterization of rat schwannoma-Schwann cell hybrids.

Sciatic nerve Schwann cells from strain LEC rats, homozygous for the c form of 6-phosphogluconate dehydrogenase (6-PGD), and RN22 rat Schwannoma cells, a subclone of RN2 deficient in hypoxanthine phosphoribosyltransferase and expressing the s form of 6-PGD, were fused to produce 'RNS' hybrid clones which proliferate rapidly in a medium containing hypoxanthine, aminopterin and thymidine (HAT) and express c, s and c/s heterodimeric forms of 6-PGD. RNS cells, like both parents, maintain a high baseline activity of 2', 3'-cyclic nucleotide 3'-phosphohydrolase and, as in RN22, activity of this enzyme is further inducible by 1 mM N6, O2'-dibutyryl 3', 5'-cyclic AMP. The RNS clones resemble normal Schwann cells in the capacity to bind radioiodinated axolemmal fragments to their plasma membranes.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗
PubMed1986

Recombination between IS5 elements: requirement for homology and recombination functions.

Intermolecular recombination between two IS5 elements was measured, using bacteriophage lambda recombination vectors, and was compared to recombination between two copies of an SV40 segment cloned into the same vectors. Experiments were conducted in the presence and in the absence of RecA and Red functions, and with the recombining inserts in the same or in reversed orientation. Under all conditions, IS5 elements recombined in a manner similar to the SV40 inserts, indicating that IS-encoded functions did not confer measurable additional intermolecular recombination ability to IS5 in E. coli K-12. Bacteriophages containing reversed IS5 inserts, for which the 16 base pair (bp) termini are identical in 15 positions and which display 12 bp of uninterrupted homology, recombined at approximately the same low frequency under Rec+ and Rec- conditions, indicating that these short homologies were not good substrates for the Rec system. Bacteriophages having reversed inserts recombined better under Red+ than under Red- conditions, but the crossovers were located in nonhomologous regions flanking the element termini. This suggests that 12-bp homologies are not good substrates for the Red system.

Bacteriophage lambda↗
PubMed1986

Very short patch mismatch repair in phage lambda: repair sites and length of repair tracts.

Five amber mutations in the repressor (cI) gene of bacteriophage lambda recombine anomalously with nearby cI mutations. When any of these markers is used in four-factor crosses, cI+ recombinants that are expected to require three cross-overs occur at high frequencies. These recombinants are attributable to very-short-patch (VSP) repair of specific mismatches in DNA heteroduplexes formed during recombination between the markers flanking cI. The sites of the repair-prone mutations and the lengths of repair tracts have now been determined. Amber mutations subject to VSP repair are C to T transitions in 5'CCATGG, the sequence methylated by the product of gene dcm, and also in the related 5'CAGG or 5'CCAG sequences. Ambers arising in CAG sequences found in other contexts, or in codons other than CAG, were not subject to VSP repair. Repair tracts rarely, if ever, exceed ten nucleotides in length, and can be as short as two nucleotides. A repair-prone mutation does not stimulate recombination between flanking cI markers.

Bacteriophage lambda↗
PubMed1985

Recombination in the lambda repressor gene: evidence that very short patch (VSP) mismatch correction restores a specific sequence.

The mutation am6 in the cI gene of bacteriophage lambda is identified as a C----T transition in a 5'CCATGG sequence. In four-factor crosses of am6 with nearby mutations in cI, the frequencies of cI+ recombinants are much higher than expected from the physical distances. A very short patch (VSP) mismatch repair system is presumed to recognize am6/am+ mispairs in the heteroduplexes that accompany recombination between the outside markers. Mutation am6 is corrected to am+; correction of am+ to am6 was not detected. Clear-plaque mutation 1-1 in cI is a T----C transition in a 5'CTTGG sequence, resulting in the sequence 5'CCATGG. When 1-1 was crossed with nearby mutations in gene cI, there were no excess cI+ recombinants, which would result from repair of CCTGG (1-1) to CTTGG (cI+). However, in crosses of cI+ phages with mutation 1-1, there was an excess of cI- recombinants, indicating that cI+ was repaired to 1-1. Preferential repair does not require adenine or cytosine methylation: when repairing a mismatch, the VSP repair system apparently identifies specific mispaired bases by sequence alone.

Bacteriophage lambda↗
PubMed1984

Crosses between insertion and point mutations in lambda gene cI: stimulation of neighboring recombination by heterology.

Intragenic recombination between lambda cI point mutations and insertions was studied in four-factor crosses. In crosses between two point mutations, there is a linear relationship between recombination frequency and distance. However, in crosses between an insertion and point mutations, there is additional recombination in the regions 200 base pairs to the right and to the left of the insertion. The recombinational stimulation occurred with IS insertions and also with insertions consisting of HindIII fragments of SV40 and with a deletion that removes part of cI. This indicated that the stimulation was a result of heterology per se rather than of information encoded by the insertions. Either Rec or Red functions are sufficient for enhanced recombination near a heterology. The stimulation is attributed to more frequent resolution of recombinational intermediates in the neighborhood of a heterology. "Stalling" of migrating branches or invading strands at a heterology may increase the probability of local DNA cleavage.

Bacteriophage lambda↗
PubMed1984

Does Chi give or take?

In lytic cycle crosses with Red-Gam-lambda phage, particles were examined that had undergone an Int-mediated exchange. It was assumed that this exchange dimerized the circular lambda, making it packageable. Among these Int-mediated recombinants, particles were identified that had, in addition, enjoyed a close double exchange mediated by the RecBC pathway. Such close double exchanges indicate localized negative interference and are analogous to eukaryotic conversions that have retained parental configuration of flanking markers. These events are stimulated by Chi, a recombinator specific to the RecBC pathway. When Chi is present in only one parent in the cross, the complementary double exchange recombinants are Chi stimulated to the same degree. This behavior of Chi contrasts with that of characterized eukaryotic recombinators.

Bacterial Proteins↗
PubMed1983

Specific mismatch correction in bacteriophage lambda crosses by very short patch repair.

In crosses under rec+, red+, gam+ conditions, mutation am6 in the cI (repressor) gene of bacteriophage lambda recombines with other cI mutations much more frequently than predicted by the physical distances involved. In four-factor crosses of am6 with mutations located 22-60 base pairs to the left, cI+ recombinants that are expected to require three crossovers (triple recombinants) are more frequent than recombinants that require only one crossover. However, when am6 is crossed with large insertions in cI, which may be expected to interfere with the formation of heteroduplexes by branch migration, the frequency of cI+ triple recombinants is very low. In addition, cI+ recombinants in crosses between am6 and adjacent mutations have a high probability of retaining the flanking markers of the am6 parent. These findings suggest that am6 is particularly susceptible to mismatch repair in heteroduplexes spanning cI. A large fraction of such heteroduplexes are presumed to be the result of branch migration from crossovers occurring at some distance from am6. The absence of co-repair when am6 is crossed with adjacent cI mutations indicates that most repair tracts extend no farther than about 20 bp to either side of the mismatch. The am6 mutation arose in the glutamine codon in a CCAGG sequence, in which the central cytosines are methylated in K12 strains. Their location in methylated sequences may make certain amber mutations susceptible to a specific very short patch (VSP) repair.

Bacteriophage lambda↗
PubMed1981

A fine structure map of spontaneous and induced mutations in the lambda repressor gene, including insertions of IS elements.

Mutations at over 70 sites in the cI gene have been mapped by 4-factor crosses and assigned precise or approximate positions in the DNA sequence. 16 of 25 spontaneous mutations were insertions of IS1, IS3 or IS5 into AT-rich regions of cI. The 5-methylcytosine in the sequence Cm5CAGG is a hot spot for spontaneous cI amber mutations. Recombination frequencies between mutations were proportional to distance with the exception of amber mutations at 4 sites, including the host spot for spontaneous mutations. Mutations with a given phenotype are clustered on the genetic map. No missense mutations affecting repressor activity were found in the central one-third of cI, but 5 of 6 ind- mutations were located in this region. The amino-terminal third of the gene contains the sites of most trans-dominant cI- mutations, and of all ts mutations that result in repressors that are reversibly inactivated at high temperatures.

Bacteriophage lambda↗
PubMed1980

Biomechanics of the wrist.

The wrist joint is a complex linkage between forearm and hand which is capable of an impressive arc of motion yet retaining a remarkable degree of stability. Carpal stability is derived from numerous intra-and intercarpal ligaments in addition to closely approximated wrist flexors and extensors. Motion occurring at the carpus is predominantly biplane--radial ulnar deviation and palmar flexion and extension. The center of motion for these planes of movement is located within the proximal and palmar pole of the capitate. When painful conditions arise at the wrist, a loss of wrist motion usually follows. Occasionally a loss of volitional control over wrist extensors is noted with the abnormal recruitment of wrist flexors with finger flexor activity. When instability and pain co-exist at the wrist, deformity can arise as a result of the inherent motor imbalance noted between the 6 wrist motors. Vector force analyses disclose that the flexor carpi ulnaris is the dominant wrist motor with the least significant force being supplied by the extensor carpi radialis longus. Although wrist motion is not essential for most activities of daily living, the preservation of wrist motion is for some individuals essential for the performance of specific occupational or recreational activities.

Biomechanical Phenomena↗
PubMed1979

Heat-sensitive lambda repressors retain partial activity during bacteriophage induction.

At 43 degrees C, lambda cIts prophages are "induced" and enter the lytic cycle. Lac- lysogens containing heat-inducible lambda N- prophages were superinfected with a lambda trp/lac N+cI- phage containing a lacZ+ gene whose expression is controlled by the lambda cI product (repressor). Lysogens were then heated, and the synthesis of beta-galactosidase and release of progeny phage were measured. In lambda N- cIts2 or lambda N-cIts16 lysogens superinfected with lambda trp/lac N+, beta-galactosidase appeared earlier and was synthesized more rapidly than in superinfected lysogens containing lambda N- cIts857 prophage. Even at 45 degrees C, the cI857 repressor retained some activity. Lysogens containing other N-cIts mutant prophages producing renaturable repressors were also only partially derepressed at 43 degrees C. Partial derepression of lambda "early" transcription is sufficient for induction of lambda N+ prophages.

Bacteriophage lambda↗
PubMed1976

Mapping missense and nonsense mutation in gene cI of bacteriophage lambda: marker effects.

Amber and missense mutations in gene cI of bacteriophage lambda were mapped by reciprocal four-factor crosses, selecting recombinants between the outside markers (N amber and O amber). Distances between cI missense mutations were additive. Several cI amber mutants recombined with other cI mutations with a higher frequency than expected from the map location. Multiple exchanges in the N-O region occurred at a frequency greater than expected by chance. This "high negative interference" was especially marked in crosses with the cI amber mutations that were strong recombiners. A new ind mutation, ind2, was found near tsU51, to the left of the previously-known ind1 mutation, which is located almost in the center of gene cI. The mutation c50 maps to the right of tsU50 and c71. Mutations c60, and ts71, which differ in phenotype, are apparently at the same site.

Chromosome Mapping↗