Cloning and sequencing the gene encoding Escherichia coli ribonuclease I: exact physical mapping using the genome library.

The amino acid (aa) sequence of the N terminus of Escherichia coli RNase I was determined. A mixed oligodeoxynucleotide coding for that sequence was used to probe the 476 lambda clones of Kohara et al. [Cell 50 (1987) 495-508]. DNA from these clones carry almost the entire E. coli chromosome in overlapping segments. Two overlapping clones hybridized to the probe sequence. From one of them DNA containing the rna gene was subcloned and sequenced. The inferred protein contains 245 aa residues and has an Mr of 27,156, which agrees with earlier estimates from sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. RNase I is close to twice the size of pancreatic RNase A, but both enzymes contain eight Cys and four His; those aa are important for structure and function of RNase A. Proximal to the rna gene is a sequence that would code for a 23-aa peptide which conforms to consensus rules for signal peptides, and thus should transport this periplasmic enzyme. Sites for eight restriction enzymes had been mapped on each lambda clone. By relating to the map for that specific region, it was possible to position the rna gene exactly at 659 kb from the thr locus (time zero on a time scale of 100 min). This physical mapping gave a more precise (exact) map position based on distance than was possible using genetic mapping based on a time scale derived from conjugation, and should be applicable for mapping many other E. coli genes.

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