Retapamulin-assisted Ribo-seq reveals the alternative bacterial proteome

Unorthodox rules of extracting genetic information enable proteome expansion without increasing the genome size. The use of alternative translation initiation sites achieves this goal by allowing production of more than one protein from a single gene. Although several such examples have been serendipitously found in bacteria, genome-wide experimental mapping of alternative translation start sites has been unattainable. We found that the antibiotic retapamulin specifically arrests initiating ribosomes at start codons of the genes. Retapamulin treatment followed by Ribo-seq analysis (Ribo-RET) not only allowed mapping of conventional initiation sites at the beginning of the annotated Escherichia coli genes but, strikingly, it also revealed putative alternative internal start sites in a number of genes. Experimental evidence demonstrated that the internal start codons can be recognized by the ribosomes and direct translation initiation in vitro and in vivo. Proteins, whose translation is initiated at an internal in-frame and out-of-frame start sites, can be functionally important and contribute to the 'alternative' bacterial proteome. In addition to proteome expansion, the internal start sites may play regulatory role in gene expression. Overall design: 4 samples of Ribo-seq data were analyzed. In this study we compared the distribution of ribosome density in BWK or BL21 strains of Escherichia coli, with or without Retapamulin treatment.