Isolation and characterization of a regulatory mutant of an aminoacyl-transfer ribonucleic acid synthetase in Escherichia coli K-12

From Escherichia coli strain K28, which is temperature sensitive for growth because of a mutation in its seryl-transfer ribonucleic acid (tRNA) synthetase gene (serS), temperature-resistant mutants were selected which were found to have a fivefold higher level of seryl-tRNA synthetase than the parent strain. The "high-level" character was found to be genetically stable and is due to a mutation in a locus denoted serO. This locus was found to be very closely linked to serS on the genetic map, and the relative gene order was concluded to be serS-serO-serC. In a serO(-) strain, the normal dependence of seryl-tRNA synthetase (SerRS) activity on changes of exogenous serine concentration was not observed. In a stable heterozygous merodiploid, the serO(-) mutation is still expressed, i.e., it is cis dominant. These results strongly suggest that serO is an operator site involved in the control of the serS gene.

Close linkage of the genes serC (for phosphohydroxy pyruvate transaminase) and serS (for seryl-transfer ribonucleic acid synthetase) in Escherichia coli K-12

Escherichia coli strain K28, isolated after nitrosoguanidine mutagenesis, was found to be auxotrophic for serine. It was also temperature sensitive for growth as a result of producing an altered seryl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.11, l-serine: tRNA ligase [AMP]). The auxotrophy was caused by a mutation in the structural gene for phosphohydroxy-pyruvate transaminase (serC), which was distinct from, but closely linked to, the structural gene for seryl-tRNA synthetase (serS). We conclude that the relevant genes are in the order gal-serS-serC-aroA.

Rapid mapping of conditional and auxotrophic mutations in Escherichia coli K-12

The approximate genetic map locations of auxotrophic and conditional lethal mutations of Escherichia coli can be rapidly determined with replica plating techniques. A set of patches of 15 streptomycin-sensitive (Str(S)) Hfr strains with points of origin distributed around the map is replica plated onto a recombinant-selective plate with a lawn of Str(R) cells which carry an unmapped mutation. The map interval defined by the Hfr points of origin which are closest to the mutant locus is seen by the presence or absence of heavy patches of recombinants produced by transfer of early wild-type genes from the Hfrs. An alternative method is to replicate patches of different mutant strains (100 per plate) onto Hfr lawns; in this case more than 1,000 different mutants can be mapped in a single experiment in a few days. In this way, many types of mutations with similar phenotypes can be grouped as to approximate location on the genetic map. For ordering mutations within groups, the same replica plating methods can be used to cross F-prime derivatives of mutants with other mutants of the same group. Relative merits of these and other mapping methods of E. coli are discussed.

Genetic location of certain mutations conferring recombination deficiency in Escherichia coli

Four mutations conferring recombination deficiency (recA1, recA13, rec-12, and rec-65) have been cotransduced with cysC and pheA; consequently they lie between 53 and 50 on the standard genetic map of Escherichia coli. The four mutations show different degrees of apparent dominance in transient rec(+)/rec(-) zygotes, and the degree of apparent dominance of a particular mutation was shown to be a characteristic of that mutation, not a reflection of other genetic differences in the original rec mutant strain. All four mutations map at similar distances from cysC and pheA and, despite the different degrees of apparent dominance, all may lie in the recA gene.

Inversion of transfer modes and sex factor-chromosome interactions in conjugation in Escherichia coli

A study was made of the mating properties of an unusual system of interconvertible donor strains of Escherichia coli K-12: Ra-1, Ra-2, and RaF(+). The Ra-1 and Ra-2 strains are Hfr strains whose origins are widely separated on the chromosome and whose transfer modes proceed in the opposite direction from one another. When Ra-1 cells were mated with females, a small fraction of the donors transferred markers via the Ra-2 mode. This effect was enhanced by preconjugal ultraviolet (UV) treatment of the Ra-1 cells. Among the survivors of UV-treated Ra-1 cells, a few stable Ra-2 cells were found. When Ra-2 cells were used as the donors, some of them were found to mate via the Ra-1 mode, in analogy with the Ra-1 to Ra-2 alteration with inversion of F mentioned above. Related experiments suggested that the inversion occurs by detachment of the F factor from one Hfr origin locus, followed by reassociation of the F factor with the other Hfr origin locus. Both the Ra-1 and Ra-2 strains reverted spontaneously to an F(+) strain, called RaF(+). Cultures of RaF(+) cells were found to mate primarily according to the Ra-1 and Ra-2 transfer modes, with smaller contributions also coming from transfer modes with origins elsewhere on the chromosome in a way which is similar to the transfer of markers from a normal F(+) strain. The RaF(+) sex factor was found to be wild type, whereas the chromosome was found to carry irregularities (sex factor affinity loci) at the locations of the Ra-1 and Ra-2 origins. Only about 10% of the donor capacity of the RaF(+) strain was due to stable spontaneous Hfr cells in cultures of RaF(+) cells.

Low recombination frequency for markers very near the origin in conjugation in E. coli

By performing matings using various Hfr strains having different origins of transfer, it has been observed that male chromosomal markers located very near the origin of transfer in conjugation inE. coliappear in recombinants with a lower frequency than expected from an extrapolation of the transfer gradient. The effect is greatest for markers closest to the origin, and becomes negligible for markers transferred more than 4–5 min. after the origin. The results suggest that in the zygote a random cross-over between male and female genomes is necessary somewhere between the origin and any male marker in order for that marker to appear in a viable recombinant. It was also deduced that the entry time for the origin itself occurs after a 3–5 min. delay after mixing male and female cells together.