Revertants from RNase III negative strains of Escherichia coli

E. coli strains carrying the rnc-105 allele do not show any level of RNase III in extracts, grow slower than rnc⁺ strains at temperatures up to 45°C and fail to grow at 45°C. Revertants which can grow at 45°C were isolated. The vast majority of them still do not grow as fast as rnc⁺ strains and did not regain RNase III activity. The mutation(s) which caused them are suppressor mutations (physiological suppressors) which do not map in the immediate vicinity of the rnc gene. A few of the revertants regain normal growth, and contain normal levels of RNase III. They do not harbor the rnc-105 allele and therefore are considered to be true revertants. By using purines other than adenine it was possible to isolate rnc⁺pur^- revertants from an rnc^-pur^- strain with relative ease. They behaved exactly like the true rnc⁺ revertants isolated from rnc^- strains at 45°C. A merodiploid strain which contains the rnc⁺ gene on an episome behaves exactly like an rnc⁺ strain with respect to growth and RNA metabolism, eventhough its specific RNase III activity is about 60% of that of an rnc⁺ strain; thus the level of RNase III is not limiting in the cell. The rnc^- strains show a characteristic pattern of transitory molecules, related to rRNA, 30S, 25S, "p23" and 18S, which are not observed in rnc⁺ strains. This pattern is unchanged in rnc^- strains and in the revertants which are still lacking RNase III, regardless of the temperature in which RNA synthesis was examined (30° to 45°C). On the other hand, in the rnc⁺ strains as well as in the true revertants and the rnc⁺/rnc^- merodiploid, the normal pattern of p16 and p23 is observed at all temperatures. These findings suggest that all the effects observed in RNase III^- strains are due to pleiotropic effects of the rnc-105 allele, and that the enzyme RNase III is not essential for the viability of the E. coli cell.