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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
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↗
PubMed1976

LambdacI mutants: intragenic complementation and complementation with a cI promoter mutant.

Complementation for the maintenance of lysogeny was studied by superinfecting lambdacIts lysogens at 34 degrees C. and then heating to 43 degrees C. With certain exceptions, ts mutants with defects in the left half of the repressor complemented ts mutants with defects in the right half to produce a less heat-labile repressor (Fig. 3). All cI amber mutants failed to complement cIts mutants. The cI mutant c50 complements all ts mutants. Mutations in Pre (cy) or genes cII and cIII do not significantly affect the expression of cI by a superinfecting lambda genome in an immune lysogen. Mutants with very heat-labile repressors failed to complement lambdacy42 for the establishment of lysogeny at elevated temperatures, while those with less heat-sensitive repressors apparently did complement cy. According to a suggested model, the left side of the cI product is concerned primarily with subunit aggregation, while operator binding is the function of the right side of the oligomer.

Coliphages↗
PubMed1976

Heat-sensitive DNA-binding activity of the cI product of bacteriophage lambda.

The binding of lambda gene cI product to lambda DNA was studied at temperatures from 0 degrees C to 46 degrees C. Binding activity of the products of cIts mutants was higher at 22 degrees C than at 0 degrees C, 26 degrees C or 30 degrees C. Both cI+ and cIts products lost DNA-binding activity at 46 degrees C, but after subsequent cooling to 22 degrees C, they regained 50-100% of their activity.

Cold Temperature↗