From the experiment, shown in Figure 5 , it is evident that both the 'Fl' and the 'SP' mRNA fragments had a half-life close to the average 2 min E. coli mRNA half-life. In addition, both the pseudoknot containing constructs (10/6 and 22/6a) revealed the existence of a short mRNA fragment that was recognized only by the gene10 probe but not the lacZ5 0 and 3 0 probes (indicated by 'asterisks' in Figure 5 ). This fragment includes the transcription start in the 5 0 -end and the pseudoknot in the 3 0 -end. We suggest that the pseudoknot acts as an exonuclease barrier like the natural stem-loop structure in the 3 0 -end of the wild type lacZ transcript (37) and thereby induces a degradation intermediate of a distinct length with increased half life compared to unstructured mRNA sequences like those from construct 0. Alternatively, but not mutually exclusive, a pseudoknot acts like a rho-independent termination signal to the RNA polymerase. However, the sequences were not followed by a row of uridine residues, which would be necessary to make a stem-loop structure into a functional transcription terminator.