Analysis of the replicon region and identification of an rRNA operon on pBM400 of Bacillus megaterium QM B1551

An 18 633 bp region containing the replicon from the approximate to 53 kb p BM400 plasmid of Bacillus megaterium QM B1551 has been sequenced and charac terized. This region contained a complete rRNA operon plus 10 other potenti al open reading frames (ORFs). The replicon consisted of an upstream promot er and three contiguous genes (repM400, orfB and orfC) that could encode pu tative proteins of 428, 251 and 289 amino acids respectively. A 1.6 kb mini mal replicon was defined and contained most of repM400. OrfB was shown to b e required for stability. Three 12 bp identical tandem repeats were located within the coding region of repM400 and their presence on another plasmid caused incompatibility with their own cognate replicon. Nonsense, frameshif t and deletion mutations in repM400 prevented replication, but each mutatio n could be complemented in trans. RepM400 had no significant similarity to sequences in the GenBank database, whereas five other ORFs had some similar ity to gene products from other plasmids and the Bacillus genome. An rRNA o peron was located upstream of the replication region and is the first rRNA operon to be sequenced from B. megaterium. Its unusual location on non-esse ntial plasmid DNA has implications for systematics and evolutionary biology .