Genetic Map of the Lactose Repressor Gene (i) of ESCHERICHIA COLI

39 tonB i^- deletions were used to map 57 independently isolated i gene mutants. Methods selective for i⁺ recombinants from various types of i mutants are described. i⁺ recombination frequencies of 6 x 10^-6 to 6 x 10^-7 can be detected in the different selective systems. Twenty i^s mutations and 12 i^-d mutations map in distinct but different regions of the gene. i^-_sus mutations are scattered over the gene.     

Mapping of the polA Locus of ESCHERICHIA COLI K12: Genetic Fine Structure of the Cistron

The close linkage of the glnA gene with polA was exploited to construct a fine structure map of polA by means of generalized transduction with phage P1. Nine different polA^- alleles were mapped by recombinational crosses. The results indicate a gene order consistent with previous observations (Kelley and Grindley 1976a; Murray and Kelley 1979). Three mutations, polA5, polA6 and polA12 map within the "carboxy-terminal" or "large-fragment" portion of the gene in unambiguous order. Four alleles, known to affect the "aminoterminal" portion of the gene, polA107, polA214, polA480ex and polA4113, appear to be closely linked with certain ambiguities in their exact order. All four of these mutations are known to alter the 5''→3'' exonuclease activity of DNA polymerase I and three of them result in the conditional lethal polA^- phenotype. The polA1 nonsense mutation maps between these two groups in a position consistent with its known effect, production of an amber fragment that includes the 5''→3'' exonuclease. The final allele, resA1, is another nonsense mutation that maps at the extreme "amino-terminus" of the cistron.——A number of control experiments were conducted to determine the effects of polA^- mutations on the P1-mediated recombinational event. These experiments indicated that abortive transduction occurs quite frequently, but the formation of abortive transductants and segregation of unselected transduced markers among daughter progeny is like that observed by other investigators. There was no evidence that any individual polA^- allele behaved in an exceptional fashion during recombination.     

A Dominant Constitutive phoR Mutation in ESCHERICHIA COLI

A dominant constitutive mutation of the phoR locus controlling alkaline phosphatase synthesis in Escherichia coli is described. Its phenotype can be explained by the production of a poisonous subunit of the phoR gene product. The phoR gene product is inferred to consist of at least 3 or 4 subunits.     

Frequency-Dependent Selection for Plasmid-Containing Cells of ESCHERICHIA COLI

Colicin-producing plasmid-containing cells of E. coli exhibit frequency-dependent selection when grown in glucose-limited continuous culture with the corresponding plasmid-free strain. The bases of this frequency-dependent effect are shown to be (1) the lower growth rate of the plasmid-containing strain under these conditions, and (2) the production of colicin, which attenuates the growth rate of the plasmid-free strain. These results are discussed in relationship to the maintenance of genetic variation in prokaryotes.     

BIOTYPING CONFIRMS A NEARLY CLONAL POPULATION STRUCTURE IN ESCHERICHIA COLI

A. reference collection of 72 natural isolates of Escherichia coli (the ECOR collection) has been examined with respect to eight metabolic capabilities (biotype characters) plus motility and resistance or sensitivity to five common antibiotics. Data from biotype characters were analyzed by means of unweighted pair-group cluster analysis, and the genetic variation among the strains defines three major clusters of strains with substantial variation within each cluster but greater genetic similarity of strains within a cluster than between clusters. These clusters define an infraspecific population structure in E. coli, which reflects the predominantly clonal mode of reproduction in this organism. The clusters identified by the biotype characters are in good agreement with those resulting from an analysis of 11 enzyme polymorphisms (allozymes) among the strains, and these are in good agreement with the infraspecific structure detected by factor analysis of allozyme data. The clusters of strains also differ in several genetic characteristics that are independent of those used in making the classification.