DNA sequencing and expression of the gene rnb encoding Escherichia coli ribonuclease II.

The Escherichia coli ribonuclease II (RNase II) is an exonuclease involved in mRNA degradation that hydrolyses single-stranded polyribonucleotides processively in the 3' to 5' direction. Sequencing of a 2.2 kb MseI-RsaI fragment containing the rnb gene revealed an open reading frame of 1794 nucleotides that encodes a protein of 598 amino acid residues, whose calculated molecular mass is 67,583 Da. This value is in good agreement with that obtained by sodium dodecyl sulphate/polyacrylamide gel electrophoresis of polypeptides synthesized by expression with the T7 RNA polymerase/promoter system. This system was also used to confirm the correct orientation of rnb. Translation initiation was confirmed by rnb-lacZ fusions. The mRNA start site was determined by S1 nuclease mapping. Two E. coli mutants harbouring different rnb alleles deficient in RNase II activity were complemented with the expressed fragment carrying the rnb gene.