Processing of Spacer tRNAs from rRNA Transcripts of Escherichia coli

Abstract

In Escherichia coli, the genes for 16S, 23S, and 5S RNA are cotranscribed from seven operons (Kiss et al. 1977; for review, see Nomura et al. 1977). A primary transcript, 30S pre-rRNA, accumulates in mutants that lack functional RNase III (Dunn and Studier 1973; Nikolaev et al. 1973). In addition to the sequences for the mature rRNAs, this 30S pre-rRNA contains the sequences for a number of tRNAs (Lund and Dahlberg 1977). In four rRNA operons, the gene for tRNA₂^Glu is located between the 16S and 23S rRNA genes; in the other three operons this spacer region contains the genes for both tRNA₁^Ile and tRNA_1B^Ala (Lund et al. 1976; Morgan et al. 1977; Ikemura and Nomura 1977). The biological significance of this genetic organization is unclear.

In cells containing functional RNase III, 30S pre-rRNA is not observed; presumably precursors to the RNAs and tRNAs are normally generated by processing of the nascent 30S pre-rRNA concurrently with its transcription (Hamkalo and Miller 1973; for reviews, see Pace 1973; Perry 1976). The total number and order of reactions involved in the eventual maturation of these precursors is currently unknown, but a likely presumption is that enzymes that process other cell tRNAs are involved in the maturation of spacer tRNAs. Furthermore, since RNase III strains are viable and do produce mature, functional ribosomes (Kindler et al. 1973), it seems probable that enzymes involved in the processing of tRNA could also provide an alternative route to rRNA maturation (Gegenheimer et al. 1977; Gegenheimer and...