CONSERVATION AND EVOLUTION OF THE RPSU DNAG RPOD MACROMOLECULAR-SYNTHESIS OPERON IN BACTERIA

The macromolecular synthesis (MMS) operon contains three essential gen es (rpsU, dnaG, rpoD) whose products (S21, primase, sigma-70) are nece ssary for the initiation of protein, DNA, and RNA synthesis respective ly. PCR amplifications with primers complementary to conserved regions within these three genes, and subsequent DNA sequencing of rpsU-dnaG PCR products, demonstrate that the three genes appear to be contiguous in 11 different Gram-negative species. Within the Gram-negative enter ic bacterial lineage, the S21 amino acid sequence is absolutely conser ved in 10 species examined. The putative nut(eq) antiterminator sequen ce in rpsU consists of two motifs, boxA and boxB, conserved in primary sequence and secondary structure. The terminator sequence, T1, locate d between rpsU and dnaG is conserved at 31 positions in nine enterobac terial species, suggesting the importance of primary sequence in addit ion to secondary structure for transcription termination. The intergen ic region between rpsU and dnaG varies in size owing to the presence o r absence of the Enterobacterial Repetitive Intergenic Consensus (ERIC ) DNA element. The rpoD gene contains rearrangements involving a diver gent sequence, although two carboxy-terminal regions which encode func tional domains are conserved in primary sequence and spacing. Our data suggest that primary sequence divergence and DNA rearrangements in bo th coding and non-coding sequences account for the interspecies variat ion in operon structure. However, MMS operon gene organization and cis -acting regulatory sequences appear to be conserved in diverse bacteri a.