Trans-translation mediated by Bacillus subtilis tmRNA

Trans-translation, in which a ribosome switches between translation of an mRNA and a tmRNA, produces a chimera polypeptide of an N-terminal truncated polypeptide and a C-terminal tag-peptide encoded by tmRNA. One of the tmRNA binding proteins, a ribosomal protein S1, has not been found in a group of Gram-positive bacteria. In this study, the trans-translation reaction with tmRNA from Bacillus subtilis belonging to this group was examined. When a truncated gene lacking a termination codon was expressed in B. subtilis, a 15-amino acid tag-peptide derived from tmRNA was identified in the C-termini of the trans-translation products. An identical tag-peptide was also found at the C-termini of the products from a truncated gene, when it was coexpressed with B. subtilis tmRNA in Escherichia coli. B. subtilis tmRNA was functional, although much less efficiently, in the in vitro poly(U)-dependent tag-peptide synthesis system of E. coli. A comparison of two bacterial tmRNAs suggests that the rule for determining the tag-initiation point on tmRNA may be the same in Gram-positive and Gram-negative bacteria.