Mutant E. coli tmRNAs. E. coli tmRNA structure is represented by a bold line. Segments of M. tuberculosis tmRNA are shown as boxes in hybrid tmRNA species. Tagging efficiencies of hybrid tmRNA derivatives were compared to the tagging efficiency of E. coli tmRNA(H8). Four pseudoknots, the open reading frame, the tRNA-like domain, helix 2 and helix 5 are denoted pk1-pk4, ORF, TLD, hp2 and hp5, respectively. The A-rich single-stranded segment connecting pk1 and ORF is marked as ‘up’. All tagging activities are shown as the mean of triplicate determinants with standard error.