Identification of transcripts from the T7gene10-PK-lacZ gene fusions. To identify the major class of transcripts from our pseudoknot containing constructs, we made a northern blot with RNA from all strains used to measure frameshift frequencies, which are those containing the upstream stop. We used three different probes hybridizing either upstream of the pseudoknot, immediately downstream of the pseudoknot or in the very end of the lacZ reading frame ( Figure 4A ). Figure 4B -D, there was an unspecific hybridization from all three probes to the 23S and 16S ribosomal RNAs. In E. coli, ribosomal RNA constitutes between 80% and 90% of total RNA depending on growth conditions and some cross-hybridization to these species is often seen in northern blots. Here, the uninduced culture in Figure 4B -D, lane '0 no IPTG', made it possible to estimate the unspecific probing to rRNA and the two bands were used as size markers on the blots.