THE EFFECTS OF MUTATIONS IN THE RPMB,G OPERON OF ESCHERICHIA-COLI ON RIBOSOME ASSEMBLY AND RIBOSOMAL-PROTEIN SYNTHESIS

The rpmB,G operon of Escherichia coli codes for proteins L28 and L33 o f the larger (50S) ribosomal subunit. Strains with mutations in this o peron can help define the roles of these proteins in ribosome synthesi s and function. One such strain, BM108, makes neither protein and is u nable to synthesize completed ribosomes; instead ribonucleoproteins ac cumulate, in the form of '30S material' and '47S particles'. However, when protein L28 is supplied from a plasmid, the growth rate, the kine tics of ribosome synthesis and the coordination of ribosomal protein s ynthesis are no different from that in wild-type organisms even though protein L33 is missing. This suggests that the latter protein can be redundant for ribosome synthesis and function. Another mutant strain, BM81, has a frameshift mutation that gives rise to an oversized protei n L28. This mutant accumulates 30S material and 47S particles during s low exponential growth. The composition of the 47S particles from stra ins BM81, BM108 and a third mutant strain, TP28, suggests that their d efining feature is the absence of L28; this is further evidence for an important role for this protein in ribosome assembly. Accumulation of ribonucleoproteins in strains BM81 and BM108 leads to some loss of th e ordinarily precise coordination of synthesis of ribosomal proteins. We describe and discuss the characteristic features of this unbalanced synthesis. (C) 1997 Elsevier Science B.V.