ISOLATION, GENETIC MAPPING AND SOME CHARACTERIZATION OF A MUTATION IN ESCHERICHIA COLI THAT AFFECTS THE PROCESSING OF RIBONUCLEIC ACID

David Apirion ¹

¹ Department of Microbiology and Immunology, Division of Biology and Biomedical Sciences, Washington University School of Medicine, St. Louis, Missouri 63110

Temperature-sensitive mutants were isolated from an rnc (RNase III-) strain of Escherichia coli, and their rRNA metabolism was analyzed on 3% polyacrylamide gels. One of these mutants was unable to produce 23S and 5S rRNAs at the nonpermissive temperature. When an rnc⁺ allele was introduced to this strain, it remained temperature sensitive. At the nonpermissive temperature, this strain could then produce 23S rRNA but was unable to make normal levels of 5S rRNA. In matings and transduction experiments, the defect in rRNA metabolism and temperature sensitivity behaved as a syndrome caused by a single point mutation, which was mapped at min 23.5 on the E. coli chromosome. This mutation probably affects an enzyme, ribonuclease E (RNase E), which introduces a cut in the nascent rRNA transcript between the 23S and the 5S rRNA cistrons. The mutation rne is recessive with respect to temperature sensitivity and the pattern of rRNA. Revertants able to grow at 43° and with normal metabolism of rRNA were isolated; genetic analysis showed that they do not contain the original rne mutation, suggesting that they were true revertants. By combining the rne mutation with an rnc mutation, double rnc rne strains were synthesized, which behaved very similarly to the original rnc strain from which the rne mutation was isolated. Such strains have RNA metabolism that is similar to that of rnc strains at permissive temperatures, but at the nonpermissive temperature they fail to synthesize p23, m23 and 5S rRNAs. Thus, the experiments reported here, together with previous studies, suggest the existence of a new processing ribonuclease activity in Escherichia coli, which is called ribonuclease E.

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