Initiation of transcription and the sequential synthesis of 16-S and 23-S ribosomal RNA in Escherichia coli

RNA synthesis in Escherichia coli, preincubated in the presence of rifampicin, on transfer to antibiotic-free medium shows a partial recovery, presumably due to fresh initiations of transcription. Incorporation of [14C]uracil into RNA during this period when chased by unlabelled uracil first appears in 16-S rRNA. Radioactivity in 23-S rRNA is detectable only for labelling periods > 40 s. It is concluded that 16-S and 23-S rRNA genes are contiguous; the former being located at the proximal end in reference to a common initiation site. The transcription time for 16-S rRNA inferred from the observed time-difference between the appearance of radioactivity in 16- and 23-S rRNA is found to be ⋍ 35 s.