IDENTIFICATION AND ANALYSIS OF THE RNC GENE FOR RNASE-III IN RHODOBACTER-CAPSULATUS

The large subunit ribosomal RNA of the purple bacterium Rhodobacter ca psulatus shows fragmentation into pieces of 14 and 16S, both fragments forming the functional equivalent of intact 23S rRNA, An RNA-processi ng step removes an extra stem-loop structure from the 23S rRNA [Kordes ,E, Jock,S,, Fritsch,J., Bosch,F, and Klug,G, (1994) J, Bacteriol,, 17 6, 1121-1127], Taking advantage of the fragmentation deficient mutant strain Fm65, we used genetic complementation to find the mutated gene responsible for this aberration, It was identified as the Rhodobacter homologue to me from Escherichia coli encoding endoribonuclease III (R Nase III), The predicted protein has 226 amino acids with a molecular weight of 25.5 kDa, It shares high homology with other known RNase III enzymes over the full length, In particular it shows the double-stran ded RNA-binding domain (dsRBD) motif essential for binding of dsRNA su bstrates. The Fm65 mutant has a frame shift mutation resulting in comp lete loss of the dsRBD rendering the enzyme inactive, The cloned Rhodo bacter enzyme can substitute RNase III activity in an RNase III defici ent E,coli strain, Contrary to E,coli, the Rhodobacter me is in one op eron together with the lep gene encoding the leader peptidase.