Hurdles to Horizontal Gene Transfer: Synonymous variation determines antibiotic resistance phenotype across species

Evidence that synonymous mutations and synonymous gene variants have fitness effects have accumulated recently. Since horizontal gene transfer represents a change in the genome of expression of the transferred gene, we hypothesized that the codon usage preferences of a horizontally transferred gene could determine the conferred fitness advantage or disadvantage, condition the immediate success of the transfer and in the longer term orient transfers. To test this hypothesis, we characterized resistance levels of synonymous variants of a gentamicin resistance gene, inserted into a broad-host range plasmid and transformed into three different bacterial species Escherichia coli, Acinetobacter baylyi and Pseudomonas aeruginosa. We revealed a strong species effect, explained in part by differences in plasmid copy number between host species. Importantly, the relative levels of resistance conferred by each synonymous variant were not conserved across species, indicating that these phenotypic effects are due to differing compatibility between the transferred variants and the receiver bacterial genomes. This species-variant interaction confirms that the codon composition of a gene can be a determinant of post-horizontal gene transfer success. However, the similarity in codon usage between the synonymous variants and the host genome only explained the phenotypic differences between variants in one species, P. aeruginosa. Further investigations of the effects of local codon usage, translation bottlenecks and internal Shine-Dalgarno sequences did not reveal common universal mechanisms across our three bacterial species and point to multiple paths leading from the synonymous sequence to phenotype and a species-specific sensitivity to these different paths.