Processing of rRNA and tRNA in Escherichia coli: Cooperation between Processing Enzymes

Abstract

rRNAs of Escherichia coli are transcribed from a number of polycistronic transcription units, each of which codes for 16S, 23S, and 5S rRNA (for a review, see Pace 1973). Genes for some tRNAs are located within rRNA transcription units, either in the central spacer or in the 3′ trailer region (Lund et al. 1976; Lund and Dahlberg 1977; Morgan et al. 1977, 1978; Gegenheimer and Apirion 1978). Processing of rRNA transcripts, then, must include tRNA processing steps. Inasmuch as intact polycistronic transcripts of rRNA are not detectable in wild-type bacterial cells, the first processing cleavage events must take place while the polycistronic precursor is still being transcribed (see Pace 1973; Gegenheimer and Apirion 1975; Gegenheimer et al. 1977; Hoffman and Miller 1977). Mutant strains of E. coli that are defective in rRNA or in tRNA processing enzymes have been isolated. These include the mutations rnc-105, rne-3071, and rnp-49, which affect the enzymes RNase III, RNase E, and RNase P, respectively (Kindler et al. 1973; Apirion and Watson 1975; Apirion 1978; Apirion and Lassar 1978; Schedl and Primakoff 1973). This discussion describes the structural analysis and in vitro processing of rRNA processing intermediates accumulated in mutant strains defective in the enzymes RNase III, RNase E, and RNase P, singly or in combinations, and demonstrates that all of these enzymes participate in production of mature cellular rRNA and tRNAs, including tRNA species not cotranscribed with rRNA. These studies also demonstrate the existence of another RNA processing enzyme, RNase F. These four enzymes...