Unusual insertion element polymorphisms in the promoter and terminator regions of the mucAB-like genes of R471a and R446b

We have previously identified umu-complementing genes on two incL/M plasmids, R471a and R446b (C. Ho et al., J, Bacteriol., 175 (1993) 5411-5419), Molecular analysis of these genes revealed that they are more structurally and functionally related to mucAB from the incN plasmid pKM101 than to other members of the previously identified Umu-like family, As a consequence, we have termed these new homologs mucAB((R471a)) and mucAB((R446b)) respectively, Interestingly, while the location of the mucAB-like genes is essentially the same in both R471a and R446b, the regions immediately flanking the mucAB-like genes are highly polymorphic, For example, 5' to mucAB((R471a)) we found an insert that appears to be a novel retroelement encoding a putative reverse transcriptase (RT). This RT is related to the reverse transcriptases encoded by group II introns but is embedded in a retron-like context, Immediately 3' to the mucAB((R471a)) locus is a putative insertion element of a sparsely-dispersed class not previously reported from enteric bacteria, Both the RT and insertion element are absent in R446b, These observations suggest that the mucAB-like genes from R471a and R446b are located within regions of the R-plasmids that perhaps were once (or still are) mobile genetic elements, Such observations might help explain the distribution of umu-like genes on R-plasmids and bacterial chromosomes, (C) 1998 Elsevier Science B.V.