Ribosomal −1 Frameshifting during Decoding ofBacillus subtilis cddOccurs at the Sequence CGA AAG

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Abstract
During translation of theBacillus subtilis cddgene, encoding cytidine deaminase (CDA), a ribosomal −1 frameshift occurs near the stop codon, resulting in a CDA subunit extended by 13 amino acids. The frequency of the frameshift is approximately 16%, and it occurs both when thecddgene is expressed from a multicopy plasmid inEscherichia coliand when it is expressed from the chromosomal copy inB. subtilis. As a result, heterotetrameric forms of the enzyme are formed in vivo along with the dominant homotetrameric species. The different forms have approximately the same specific activity. Thecddgene was cloned in pUC19 such that thelacZ′ gene of the vector followed thecddgene in the −1 reading frame immediately after thecddstop codon. By using site-directed mutagenesis of thecdd-lacZ′ fusion, it was shown that frameshifting occurred at the sequence CGA AAG, 9 bp upstream of the in-framecddstop codon, and that it was stimulated by a Shine-Dalgarno-like sequence located 14 bp upstream of the shift site. The possible function of this frameshift in gene expression is discussed.