Messenger RNA stability. The wild type lacZ mRNA half-life is close to the average mRNA half-life in E. coli (120 s) and transcription takes close to 80 s due to the length of the lacZ gene (three times longer than the average gene). Therefore, a northern blot of wild type lacZ mRNA under steady state transcription will always include a lot of unfinished native transcripts, as well as mRNAs under degradation. Here, our gene10-lacZ fusion was even longer and transcription should take $120 s. Accordingly, all induced strains included in Figure 4 show a distinct smear of mRNA fragments recognized by all three probes. In order to examine the half-life of our artificial transcripts, we made experiments where transcription from the P tac promotor was stopped due to removal of the inducer ( Figure 5 ). Two minutes after IPTG removal, any remaining smear should originate from mRNA degradation because most of the RNA polymerase should have reached the end of the gene fusion at this time.