Isolation of RNase H genes that are essential for growth of Bacillus subtilis 168

Two genes encoding functional RNase H (EC 3.1.26.4) were isolated from a gr am-positive bacterium, Bacillus subtilis 168. Two DNA clones exhibiting RNa se H activities both in vivo and in vitro were obtained from a B. subtilis DNA library. One (28.1 kDa) revealed high similarity to Escherichia coli RN ase HII, encoded by the rnhB gene. The other (33.9 kDa) was designated rnhC and encodes B. subtilis RNase HIII. The B. subtilis genome has an rnhA hom ologue, the product of which has not yet shown RNase H activity. Analyses o f all three B. subtilis genes revealed that rnhB and rnhC cannot be simulta neously inactivated. This observation indicated that in B. subtilis both th e rnhB and rnhC products are involved in certain essential cellular process es that are different from those suggested by E. coli rnh mutation studies. Sequence conservation between the rnhR and rnhC genes implies that both or iginated from a single ancestral RNase H gene. The roles of bacterial RNase H may be indicated by the single rnhC homologue in the small genome of Myc oplasma species.