Insertions of Tn 10 into an E. coli ribosomal RNA operon are incompletely polar

Insertions of the transposon Tn 10 have been obtained in an E. coli ribosomal RNA operon (rrnX). These insertions were used to determine whether present models of polarity are sufficient to explain transcriptional properties of operon which are transcribed but not translated. Most models of polarity suggest that transcription of mRNA requires simultaneous translation of nascent mRNA, or else premature termination of transcription will result. This model as stated does not encompass rrn operons. In this study three Tn 10 insertions into rrnX were characterized. All three Tn 10 insertions do not completely eliminate in vivo transcription of the most distal tRNA gene of rrnX. In ultraviolet-irradiated cells one Tn 10 insertion reduces synthesis of downstream portions of rrnX by at most 2--3 fold. Two other insertions may have more extensive polar effects, although the exact level of polarity is difficult to evaluate because these Tn 10 insertions may affect RNA maturation and stability.