Altered mRNA metabolism in ribonuclease III-deficient strains of Escherichia coli

The metabolism of mRNA from the lactose (lac) operon of Escherichia coli has been studied in ribonuclease (RNase) III-deficient strains (rnc-105). The induction lag for beta-galactosidase from the first gene was twice as long, and enzyme synthesis was reduced 10-fold in one such mutant compared with its isogenic rnc+ sister; in the original mutant strain AB301-105, synthesis of beta-galactosidase was not even detectable, although transduction analysis revealed the presence of a normal lac operon. This defect does not reflect a loss of all lac operon activity galactoside acetyltransferase from the last gene was synthesized even in strain AB301-105 but at a rate several times lower than normal. Hybridization analyses suggested that both the frequency of transcription initiation and the time to transcribe the entire operon are normal in rnc-105 strains. The long induction lag was caused by a longer translation time. This defect led to translational polarity with reduced amounts of distal mRNA to give a population of smaller-sized lac mRNA molecules. All these pleiotropic effects seem to result from RNase III deficiency, since it was possible to select revertants to rnc+ that grew and expressed the lac operon at normal rates. However, the rnc-105 isogenic strains (but not AB301-105) also changed very easily to give a more normal rate of beta-galactosidase synthesis without regaining RNase III activity or a faster growth rate. The basis for this reversion is not known; it may represent a "phenotypic suppression" rather than result from a stable genetic change. Such suppressor effects could account for earlier reports of a noninvolvement of RNase III in mRNA metabolism in deliberately selected lac+ rnc-105 strains. The ribosomes from rnc-105 strains were as competent as ribosomes from rnc+ strains to form translation initiation complexes in vitro. However, per mass, beta-galactosidase mRNA from AB301-105 was at least three times less competent to form initiation complexes than was A19 beta-galactosidase mRNA. RNase III may be important in the normal cell to prepare lac mRNA for translation initiation. A defect at this step could account for all the observed changes in lac expression. A potential target within a secondary structure at the start of the lac mRNA is considered. Expression of many operons may be affected by RNase III activity; gal and trp operon expressions were also abnormal in RNase III- strains.