Mutation and subsequent recombination events create genetic diversity, which is subjected to natural selection. Bacterial mismatch repair (MMR) deficient mutants, exhibiting high mutation and homologous recombination rates, are frequently found in natural populations. Therefore, we have explored the possibility that MMR deficiency emerging in nature has left some "imprint" in the sequence of bacterial genomes. Comparative molecular phylogeny of MMR genes from natural Escherichia coli isolates shows that, compared to housekeeping genes, individual functional MMR genes exhibit high sequence mosaicism derived from diverse phylogenetic lineages. This apparent horizontal gene transfer correlates with hyperrecombination phenotype of MMR-deficient mutators. The sequence mosaicism of MMR genes may be a hallmark of a mechanism of adaptive evolution that involves modulation of mutation and recombination rates by recurrent losses and reacquisitions of MMR gene functions.

No abstract

Thirty-six ruminant isolates of Chlamydia psittaci, previously classified as invasive or non-invasive in a mouse model of virulence, were compared by analysing A h 1 restriction patterns of the major outer-membrane protein (MOMP) gene after DNA amplification by the polymerase chain reaction. The 24 invasive isolates, although from various origins, all belonged to serotype 1 and represented a strictly homogeneous group sharing a specific MOMP-gene restriction'pattern that was not observed in the non-invasive strains. On the other hand, the 12 noninvasive strains, although all belonging to serotype 2, constituted a heterogeneous group with eight distinct MOMP-gene restriction patterns. However, all eight patterns shared a 180 bp fragment or the corresponding restricted fragments of 110 and 70 bp. MOMP-gene restriction patterns also clearly distinguished the ruminant strains from an avian C.psi#uciisolate, a Cpneumoniae isolate and two C. truchomatis isolates which were studied for comparison. The homogeneous character of the invasive C. psiffuci strains argues strongly for their genetic relatedness. Our results illustrate the usefulness of the MOMP-gene restriction mapping in typing chlamydiae.