Transcription patterns of adjacent segments on the chromosome of Escherichia coli containing genes coding for four 50 S ribosomal proteins and the β and β′ subunits of RNA polymerase

Abstract

The transcription of four distinct segments of the bacterial chromosome has been examined under different conditions of steady-state growth using specific RNA-DNA hybridization assays. The genome segments contain, respectively: (1) a cluster of genes coding for four ribosomal proteins, (2) the gene coding for the β subunit of RNA polymerase, (3) the gene coding for the β′ subunit of RNA polymerase and (4) a cluster of genes coding for 15 ribosomal proteins and the α subunit of RNA polymerase. The first three segments are contiguous and located near 88 minutes, whereas the fourth segment is distinctly separate and located near 72 minutes on the Escherichia coli linkage map. The amount of RNA (measured as a fraction of the total cellular RNA) which hybridized to the DNA segments coding for ribosomal proteins was found to increase with the steady-state growth rate of the bacterium. In contrast, the amount of RNA that was homologous to the genes coding for the β and β′ subunits of RNA polymerase was invariant with the growth rate. The amount of β′ messenger RNA was apparently greater than the amount of β mRNA, suggesting that β′ mRNA is more stable than β mRNA during the exponential phase growth. The amount and frequency of transcription of these DNA segments was determined by hybridization with pulse-labeled RNA. The results of these experiments surprisingly indicated that the four segments were transcribed proportionately and independent of the bacterial growth rate. The ribosomal protein gene clusters at 88 minutes and 72 minutes were transcribed with about equal frequency; this indicates that the transcription of distinctly separate gene clusters can be regulated co-ordinately. The DNA segments coding for the β and β′ subunits of RNA polymerase were transcribed with equal frequency, and their transcription was co-ordinate with, but only one-fourth or one-fifth as frequent as transcription of the adjacent ribosomal protein genes. This observation is consistent with the idea that the genes coding for the β and β′ subunits are cotranscribed, and further suggests that regulation of the β and β′ subunit genes is intimately related to the regulation of the adjacent ribosomal protein genes.