RRNA and tRNA Processing Signals in the rRNA Operons of Escherichia coli

Abstract

It has been known for some time that the three rRNAs of Escherichia coli are transcribed in the order 16S, 23S, and 5S into a single precursor molecule, which is then cleaved and trimmed by a host of specific ribonucleases. Precursors of bacterial tRNAs have long been the subject of studies aimed at pinpointing those aspects of their structures that dictate the precise processing events required to fashion a functional tRNA molecule. However, only relatively recently were tRNA genes identified within the E. coli rRNA operons (Lund et al. 1976; see also Morgan et al., this volume). The presence of such tRNA genes requires the processing pathways for both types of stable RNA molecules to be productively coordinated within a growing cell. To examine rRNA and tRNA processing sites in the primary rRNA transcript of E. coli, we have determined selected sequences from two of the seven E. coli rRNA operons. These are rrnD and rrnX (a hybrid operon) carried by the transducing phages λ daroE and λd ilv5 , respectively. So far we have analyzed the region preceding the 16S gene, the entire 16S–23S spacer, and the region between the 23S and 5S gene. The location of these DNA stretches in the two rRNA operons is shown in Figure 1. Features involved in the recognition of the rRNA transcript by known rRNA and tRNA processing enzymes are summarized below. A description of the DNA analysis, as well as a more thorough discussion of the sequences obtained, has been presented elsewhere (Young...