We recently identified RrmJ (alias FtsJ), the first encoded protein of the rrmJ-hflB heat shock operon, as an Um(2552) methyltransferase of the 23S rRNA. We now report that the rrmJ-deficient strain exhibits growth and translational defects compared to the wild-type strain. Growth rates of the rrmJ mutant are decreased at both low and high temperatures. Protein synthesis activity is reduced up to 65% when S-30 rrmJ mutant extracts are tested in a coupled in vitro transcription/translation assay. In vitro methylation of these extracts by RrmJ partially restores protein synthesis activity. Polysome profile analysis of the rrmJ strain reveals an increase in the proportion of free 30S and 50S subunits at both 30 and 42 degrees C. These results suggest that the RrmJ-catalyzed methylation of Um(2552) in 23S RNA strengthens ribosomal subunit interactions, increases protein synthesis activity, and improves cell growth rates even at non-heat shock. temperatures.