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.