Evaluation of the Role of RecA Protein in Plant Virulence with recA Mutants of Xanthomonas campestris pv. campestris

Xanthomonas campestris pv. campestris NRRL B1459 recA mutants were isolated by recombination with an interrupted Rhizobium etli recA gene and selection of double recombinants. The mutants were impaired in homologous genetic recombination and in DNA repair as judged by their sensitivity to methyl-methane-sulfonate and to UV irradiation; these defects are complemented in trans by the R. etli recA gene. Virulence of X. campestris pv. campestris NRRL B1459 to cabbage is considerably diminished by the recA mutation. The recA mutation is not correlated with the frequency of occurrence of a genetic rearrangement that affects chemotaxis, plant virulence, and xanthan gum production.

Recombination-deficient mutations and thymineless death in Escherichia coli K12: reciprocal effects of recBC and recF and indifference of recA mutations

In an approach to characterizing the nature of the lethal event in thymineless death (TLD), rec mutants of Escherichia coli K12 were examined for their sensitivity to TLD. The recB21 and recC22 mutations sensitized cells of the AB1157 line to TLD but not cells of the HF4733 line. This increased sensitivity was not suppressed substantially by either sbcB15 or xonA1 mutation. In contrast, a recF mutation appeared to make cells more resistant to TLD than rec+ cells. Three different recA alleles were shown not to affect TLD appreciably. These results not only provide further support for the view that the site of the lethal event in TLD is cellular DNA, but also strongly suggest the involvement of the recBC and recF gene products in TLD. The apparent indifference of recA mutation implies that the conventional recombination and repair pathways per se are not involved in TLD and that the hypothetical lethal damage to DNA may be unique in nature.

THE DISTRIBUTION OF CROSSOVERS ALONG UNREPLICATED LAMBDA BACTERIOPHAGE CHROMOSOMES

ABSTRACT The distribution of crossovers along unreplicated chromosomes of bacteriophage lambda has been examined by determining the density distributions and genotypes of particles in the progenies of crosses of density-labeled by ordinary parents in the presence of genetic blocks to replication. The Red and Rec systems combined produce crossovers primarily near the ends (especially the right end) of the chromosome. Removal of the generalized lambda recombination functions by red and gam mutations results in loss of these terminal crossovers; coupled with this loss is a disappearance of the differential dependence of recombination frequencies in terminal and central intervals on DNA synthesis. Removal of the bacterial system by a recA mutation results in severe depression of crossing over among unreplicated phage, with the few recombinants produced by the lambda system occurring near the right end.