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 pl asmids, R471a and R446b (C. Ho et al., J, Bacteriol., 175 (1993) 5411- 5419), Molecular analysis of these genes revealed that they are more s tructurally and functionally related to mucAB from the incN plasmid pK M101 than to other members of the previously identified Umu-like famil y, 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 polym orphic, For example, 5' to mucAB((R471a)) we found an insert that appe ars to be a novel retroelement encoding a putative reverse transcripta se (RT). This RT is related to the reverse transcriptases encoded by g roup II introns but is embedded in a retron-like context, Immediately 3' to the mucAB((R471a)) locus is a putative insertion element of a sp arsely-dispersed class not previously reported from enteric bacteria, Both the RT and insertion element are absent in R446b, These observati ons 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 di stribution of umu-like genes on R-plasmids and bacterial chromosomes, (C) 1998 Elsevier Science B.V.