Previous evidence has indicated that the 16S rRNA genes in certain species of Aeromonas may have a history of lateral transfer and recombination. A comparative analysis of patterns of 16S nucleotide sequence polymorphism among species ofRhizobium andAgrobacterium was conducted to determine if there is similar evidence for chimeric 16S genes in members of the Rhizobiaceae. Results from phylogenetic analyses and comparison of patterns of nucleotide sequence polymorphism in portions of rhizobial 16S genes revealed the same type of segment-dependent polymorphic site partitioning that was previously reported for Aeromonas. These results support the hypothesis that certain 16S segments in rhizobia may have a history of lateral transfer and recombination.Abbreviations: 16S rRNA -16S ribosomal ribonucleic acid, 16S -the 16S rRNA gene.

The sequence of aceK, which codes for the regulatory catalytic enzyme isocitrate dehydrogenase kinase/phosphatase (IDH K/P), and sequences of the 5′ flanking region and part or all of the 3′ flanking region were determined for 32 strains of Salmonella enterica and Escherichia coli. In E. coli, the aceK gene was 1734 bp long in 13 strains, but in three strains it was 12 bp shorter and the stop codon was TAA rather than TGA. Strains with the shorter aceK lacked an open reading frame (f728) downstream between aceK and iclR that was present, in variable length, in the other strains. Among the 72 ECOR strains, the truncated aceK gene was present in all isolates of the B2 group and half of those of the D group. Other variant conditions included the presence of IS1 elements in two strains and large deletions in two strains. The aceK-aceA intergenic region varied in length from 48 to 280 bp in E. coli, depending largely on the number of repetitive extragenic palindromic (REP) sequences present. Among the ECOR strains, the number of REP elements showed a high degree of phylogenetic association, and sequencing of the region in the ECOR strains permitted partial reconstruction of its evolutionary history. In S. enterica, the normal length of aceK was 1752 bp, but three other length variants, ranging from 1746 to 1785 bp, were represented in five of the 16 strains examined. The flanking intergenic regions showed relatively minor variation in length and sequence. The occurrence of several nonrandom patterns of distribution of polymorphic synonymous nucleotide sites indicated that intragenic recombination of horizontally exchanged DNA has contributed to the generation of allelic diversity at the aceK locus in both species.