Transient erythromycin resistance phenotype associated with peptidyl-tRNA drop-off on early UGG and GGG codons

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

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

^* To whom correspondence should be addressed. Email: Leif.Isaksson{at}gmt.su.se.

	Abstract

Expression of minigenes encoding tetra- or penta-peptides MXLX or MXLXV (E-peptides), where X is a non-polar amino acid, renders cells erythromycin resistant whereas expression of mini-genes encoding tri-peptide 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 +4 or +5, without a following stop codon, are associated with an erythromycin resistance phenotype upon gene induction. Our results suggest that, while a stop codon at +4 gives a tri-peptide product (MIL) and erythromycin sensitivity, UGG or GGG codons at the same position give a tetra-peptide 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.