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Journal of Bacteriology, December 2007, p. 8993-9000, Vol. 189, No. 24
0021-9193/07/$08.00+0     doi:10.1128/JB.01004-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Transient Erythromycin Resistance Phenotype Associated with Peptidyl-tRNA Drop-Off on Early UGG and GGG Codons

Mirjana Mavanin,¹ Ernesto I. Gonzalez de Valdivia,¹ David H. Ardell,² and Leif A. Isaksson^1*

Department of Genetics, Microbiology and Toxicology, Stockholm University, Svante Arrhenius väg 16F, 106 91 Stockholm, Sweden,¹ Linnaeus Centre for Bioinformatics, Uppsala University, Box 598, Biomedical Center, SE-751 24 Uppsala, Sweden²

Received 25 June 2007/ Accepted 28 September 2007

Expression of minigenes encoding tetra- or pentapeptides MXLX or MXLXV (E peptides), where X is a nonpolar amino acid, renders cells erythromycin resistant whereas expression of minigenes encoding tripeptide MXL does not. By using a 3A' reporter gene system beginning with an E-peptide-encoding sequence, we asked whether the codons UGG and GGG, which are known to promote peptidyl-tRNA drop-off at early positions in mRNA, would result in a phenotype of erythromycin resistance if located after this sequence. We find that UGG or GGG, at either position +4 or +5, without a following stop codon, is associated with an erythromycin resistance phenotype upon gene induction. Our results suggest that, while a stop codon at +4 gives a tripeptide product (MIL) and erythromycin sensitivity, UGG or GGG codons at the same position give a tetrapeptide product (MILW or MILG) and phenotype of erythromycin resistance. Thus, the drop-off event on GGG or UGG codons occurs after incorporation of the corresponding amino acid into the growing peptide chain. Drop-off gives rise to a peptidyl-tRNA where the peptide moiety functionally mimics a minigene peptide product of the type previously associated with erythromycin resistance. Several genes in Escherichia coli fulfill the requirements of high mRNA expression and an E-peptide sequence followed by UGG or GGG at position +4 or +5 and should potentially be able to give an erythromycin resistance phenotype.

Published ahead of print on 19 October 2007.