Ribosomal RNA genes in Bacillus subtilis. Evidence for a cotranscription mechanism

The analysis of the transcriptional mechanism of the ribosomal RNA genes in Bacillus subtilis was undertaken by a study of the rRNA chain elongation in the presence of rifampicin. The residual RNA synthesis after the addition of rifampicin and [3H] uridine to exponentially growing cells has shown that 56% of the radioactivity incorporated into total RNA belongs to the unstable fraction and 44% to the fraction containing mature rRNA and tRNA. Such study allowed an estimation of the half-life of messenger RNAs as being approximately 2 min. The analysis of the transcription pattern of the ribosomal RNA genes, as measured by the amount of radioactivity found in the ribosomal subunits, was complicated by a contamination of the 30 S subunits by 50 S subunits. A contamination of approximately 15% was estimated by polyacrylamide gel electrophoresis and competitive hybridization. The ratios of incorporated radioactivity at zero time when drug and label were concomitantly added ranged between 5.4-6.0, after correction for this contamination. The decay of the 23 S rRNA followed a straight line which became parabolic in its final portion. These results, and theoretical considerations on the lag of rifampicin action and on the variance of the specific activity of the nucleotide pool at the very early times of the experimental observation, favor the interpretation that the 16 and 23 S rRNA genes in B. subtilis belong to the same transcriptional unit, being cotranscribed, in that order, by the same molecule of RNA polymerase. The transcriptional times of the 16 and 23 S rRNA genes were estimated as being 30 and 60 s, respectively.