Linkage distortion following conjugational transfer of sbcC+ to recBC sbcBC strains of Escherichia coli

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Conjugational recombination in Escherichia coli: genetic analysis of recombinant formation in Hfr x F- crosses

The formation of recombinants during conjugation between Hfr and F- strains of Escherichia coli was investigated using unselected markers to monitor integration of Hfr DNA into the circular recipient chromosome. In crosses selecting a marker located approximately 500 kb from the Hfr origin, 60-70% of the recombinants appeared to inherit the Hfr DNA in a single segment, with the proximal exchange located > 300 kb from the selected marker. The proportion of recombinants showing multiple exchanges increased in matings selecting more distal markers located 700-2200 kb from the origin, but they were always in the minority. This effect was associated with decreased linkage of unselected proximal markers. Mutation of recB, or recD plus recJ, in the recipient reduced the efficiency of recombination and shifted the location of the proximal exchange(s) closer to the selected marker. Mutation of recF, recO or recQ produced recombinants in which this exchange tended to be closer to the origin, though the effect observed was rather small. Up to 25% of recombinant colonies in rec+ crosses showed segregation of both donor and recipient alleles at a proximal unselected locus. Their frequency varied with the distance between the selected and unselected markers and was also related directly to the efficiency of recombination. Mutation of recD increased their number by twofold in certain crosses to a value of 19%, a feature associated with an increase in the survival of linear DNA in the absence of RecBCD exonuclease. Mutation of recN reduced sectored recombinants in these crosses to approximately 1% in all the strains examined, including recD. A model for conjugational recombination is proposed in which recombinant chromosomes are formed initially by two exchanges that integrate a single piece of duplex Hfr DNA into the recipient chromosome. Additional pairs of exchanges involving the excised recipient DNA, RecBCD enzyme and RecN protein, can subsequently modify the initial product to generate the spectrum of recombinants normally observed.

The phage lambda orf gene encodes a trans-acting factor that suppresses Escherichia coli recO, recR, and recF mutations for recombination of lambda but not of E. coli

Bacteriophage lambda can recombine in recBC sbcB sbcC mutant cells by using its own gene orf, the Escherichia coli recO, recR, and recF genes, or both. Expression of an orf-containing plasmid complements the recombination defects of orf mutant phage. However, this clone does not complement a recO mutation for conjugational recombination or recO, recR, or recF mutations for repair of UV-induced DNA damage. A plasmid clone of orf produces a protein with an apparent molecular mass of 15 kDa.

Formation of a gated channel by a ligand-specific transport protein in the bacterial outer membrane

The ferric enterobactin receptor (FepA) is a high-affinity ligand-specific transport protein in the outer membrane of Gram-negative bacteria. Deletion of the cell-surface ligand-binding peptides of FepA generated mutant proteins that were incapable of high-affinity uptake but that instead formed nonspecific, passive channels in the outer membrane. Unlike native FepA, these pores acted independently of the accessory protein TonB, which suggests that FepA is a gated porin and that TonB acts as its gatekeeper by facilitating the entry of ligands into the FepA channel. The sequence homology among TonB-dependent proteins suggests that all ligand-specific outer membrane receptors may function by this gated-porin mechanism.

Genetic analysis of recombination in prokaryotes

Bacteria provide a simple system for the genetic analysis of homologous recombination. More than twenty genes have been identified in Escherichia coli. The enzymatic activities associated with the products of many of these genes have been revealed by studies with model DNA substrates. It is now possible to pair homologous molecules in vitro and process these through defined intermediates into mature recombinants of the types predicted by genetic crosses.