STRATEGIES USED BY PATHOGENIC AND NONPATHOGENIC MYCOBACTERIA TO SYNTHESIZE RIBOSOMAL-RNA

One rRNA operon of all mycobacteria studied so far is located downstre am from a gene thought to code for the enzyme UDP-N-acetylglucosamine carboxyvinyl transferase (UNAcGCT), which is important to cell wall sy nthesis. This operon has been designated rrnA(f) for fast-growing myco bacteria and rrnA(s) for slow growers. We have investigated the upstre am sequences and promoter activities of rrnA(f) operons of typical fas t growers which also possess a second rrn (rrnB(f)) operon and of the rrnA operons of the fast growers Mycobacterium abscessus and Mycobacte rium chelonae, which each have a single rrn operon per genome. These f ast growers have a common strategy for increasing the efficiency of tr anscription of their rrnA operons, thereby increasing the cells' poten tial for ribosome synthesis. This strategy involves the use of multipl e (three to five) promoters which may have arisen through successive d uplication events, Thus we have identified a hypervariable multiple pr omoter region (HMPR) located between the UNAcGCT gene and the 16S rRNA coding region. Two promoters, P1 and PCL1, appear to play pivotal rol es in mycobacterial rRNA synthesis; they are present in all of the spe cies examined and are the only promoters used for rRNA synthesis by th e pathogenic slow growers. P1 is located within the coding region of t he UNAcGCT gene, and PCL1 has a characteristic sequence that is relate d to but distinct from that of the additional promoters. In fast-growi ng species, P1 and PCL1 produce less than 10% of rRNA transcripts, so the additional promoters found in the HMPR are important in increasing the potential for rRNA synthesis during rapid growth, In contrast, rr nB operons appear to be regulated by a single promoter; because less d ivergence has taken place, rrnB appears to be younger than rrnA.