Homologous recombination between the tuf genes of Salmonella typhimurium.

The genes coding for the translation factor EF-Tu, tufA and tufB are separated by over 700 kb on the circular chromosome of Salmonella typhimurium. The coding regions of these genes have 99% identity at the nucleotide level in spite of the presumed ancient origin of the gene duplication. Sequence comparisons between S. typhimurium and Escherichia coli suggest that within each species the two tuf genes are evolving in concert. Here we show that each of the S. typhimurium tuf genes can transfer genetic information to the other. In our genetic system the transfers are seen as non-reciprocal, i.e. as gene conversion events. However, the mechanism of recombination could be reciprocal, with sister chromosome segregation and selection leading to the isolation of a particular class of recombinant. The amount of sequence information transferred in individual recombination events varies, but can be close to the entire length of the gene. The recombination is RecABCD-dependent, and is opposed by MutSHLU mismatch repair. In the wild-type, this type of recombination occurs at a rate that is two or three orders of magnitude greater than the nucleotide substitution rate. The rate of recombination differs by six orders of magnitude between a recA and a mutS strain. Mismatch repair reduces the rate of this recombination 1000-fold. The rate of recombination also differs by one order of magnitude depending on which tuf gene is donating the sequence selected for. We discuss three classes of model that could, in principle, account for the sequence transfers: (1) tuf mRNA mediated recombination; (2) non-allelic reciprocal recombination involving sister chromosomes; (3) non-allelic gene conversion involving sister chromosomes, initiated by a double-strand break close to one tuf gene. Although the mechanism remains to be determined, the effect on the bacterial cells is tuf gene sequence homogenisation. This recombination phenomenon can account for the concerted evolution of the tuf genes.