The transfer RNA processing defect in Escherichia coli strains BN and CAN is not due to a mutation in RNAase D or RNAase II

Escherichia coli strains BN and CAN are unable to support the growth of bacteriophage T4 psu₁⁺-amber double mutants. For strain BN, this phenotype has been attributed to a defect in 3′ processing of the precursor to psu₁⁺ tRNA^Ser. Since RNAase D and RNAase II are the only well-characterized 3′ exoribonucleases to be implicated in tRNA processing, the status of these activities and their genes in the mutant strains was investigated. Although extracts of strains BN and CAN were defective for hydrolysis of the artificial tRNA precursor, tRNA-C-U, these strains contained normal levels of RNAase D and RNAase II, and purified RNAase D or RNAase II could only partially complement the mutant extracts. Introduction of the wild-type RNAase D gene into strains BN and CAN did not correct the mutant phenotype. Likewise, strains defective in RNAase D and/or RNAase II plated T4psu₁⁺-amber phage normally. These results indicate that the tRNA processing defect in strains BN and CAN is not due to a mutation in either RNAase U or RNAase II. The possibility that the mutation in these strains affects another exoribonuclease or a factor influencing the activity and specificity of RNAase D or RNAase II is discussed.