In vivo transcription of E. coli genes coding for rRNA, ribosomal proteins and subunits of RNA polymerase: Influence of the stringent control system

Abstract

The synthesis of RNA and protein components of the ribosome is subject to stringent control. Elicitation of stringent or relaxed control occurs following limitation of aminoacylated tRNA in isogenic relA+ and relA- strains respectively. We examined the transcription of rRNA genes and of three additional mRNA producing segments of the bacterial chromosome. These segments code respectively for (i) 15 ribosomal proteins (r-protein) and the α subunit of RNA polymerase, (ii) four 50S r-proteins and (iii) the β and β′ subunits of RNA polymerase. The first segment is located near 72 min whereas the second and third segments are contiguous with each other and located near 88 min. We have shown that when the relA+ strain was limited for valyl tRNA the unusual nucleotide guanosine tetraphosphate (ppGpp) accumulates and transcription of genes coding for rRNA and r-proteins was reduced; transcription of the genes coding for the β and β′ subunits of RNA polymerase was not affected. When we limited the relA- strain ppGpp failed to accumulate and the transcription of rRNA and r-protein genes was enhanced; in contrast transcription of the β and β′ genes was not enhanced. We have concluded that the stringent control system directly or indirectly regulates the genes coding for RNA and protein components of the ribosome at the level of transcription. The influence of this control system on the transcription of the two distinct and separate clusters of r-protein genes was identical. In addition we have shown that transcription of the genes for the β and β′ subunits of RNA polymerase was distinctly different from transcription of r-protein genes and not subject to stringent control.