METABOLISM OF RIBOSOMAL RNA IN MUTANTS OF ESCHERICHIA COLI DOUBLY DEFECTIVE IN RIBONUCLEASE III AND THE TRANSCRIPTION TERMINATION FACTOR RHO

To determine if proteins RNase III and rho, both of which can determine the 3' ends of RNA molecules, can complement each other, double mutants defective in these two factors were constructed. In all cases (four rho mutations tested) the double mutants were viable at lower temperatures, but were unable to grow at higher temperatures at which both of the parental strains grew. Genetic analyses suggested that the combination of the rnc rho (RNase III-Rho-) mutations was necessary and probably sufficient to confer temperature sensitivity on carrier strains. Physiological studies showed that synthesis and maturation of rRNA, which is greatly affected by RNase III, as well as other RNAs, was indistinguishable in rnc rho strains as compared to rnc rho  + strains, thus suggesting that RNase III and rho do not complement one another in determining the 3' ends of RNA molecules. In rnc rho strains, however, the newly synthesized rRNA failed to accumulate. Thus, decay of rRNA could be the reason for the temperature sensitivity of the double mutant strains. These experiments suggest that RNase III and rho can both protect rRNA from degradation by cellular ribonucleases. They also point to the possibility that the nucleotide sequences involved in the determination of the 3' ends of RNA molecules by these two factors are not identical.