A comprehensive computational analysis investigating the relationships between phage codon usage, infection style, and number of tRNA genes

Abstract

It has been known for decades that bacteriophages encode tRNA genes, but their function and the factors contributing to their acquisition and retention are unclear. Although tRNAs are found in a variety of phages infecting a variety of bacteria, many large-scale computational studies investigating tRNA acquisition and retention in phages are specific to Mycobacterium phages; however, these findings may not be representative of other phages or bacteria. This work uses a broader sampling of phages and hosts to investigate the relationships between codon usage bias, infection cycle, and tRNA gene numbers in phage genomes. We analyzed 154 phages infecting 7 host genera, including Gram-negative (Escherichia, Shigella, Salmonella) and Gram-positive (Bacillus, Lactobacillus, Staphylococcus, Mycobacterium) bacteria. Phages included temperate and virulent representatives, plus a range of tRNA numbers and morphologies. All phages and hosts were analyzed using four metrics: GC content, Effective Number of Codons, Relative Synonymous Codon Usage, and tRNA Adaptation Index. On a global scale, virulent phages with many tRNA genes show greater differences in codon usage and codon adaptation compared to their respective hosts. Gram-negative bacteria and their phages generally exhibit greater differences in codon usage compared to Gram-positive bacteria and their phages. Phages infecting Gram-negative hosts also tend to encode more tRNA genes. In nearly all genus-level comparisons, Mycobacterium phages were different from any other host and from global patterns. This suggests previous computational studies performed in Mycobacterium phages are likely not applicable on a global scale or to phages infecting other host genera. AUTHOR SUMMARYBacteriophages, or phages, are viruses infecting bacteria. They are abundant in all environments, yet how they interact with their bacterial hosts is still not well-understood. Like other viruses, phages must rely on the host translational components to replicate and form new phage particles; and similarly to other parasites, phages have genomes that differ significantly from their hosts in terms of composition. In this work, we explore the relationship between phage lifestyle, number of tRNA genes encoded, and genome differences from the host using a variety of phages and their associated hosts. Phages can be either virulent (do not integrate into the host genome) or temperate (capable of integrating into the host genome), with differences from the host genome more pronounced in virulent phages. There are many phages that also carry tRNA genes, and having higher numbers of tRNAs is associated with larger differences from the host genome. The findings here indicate that virulent phages carrying large numbers of tRNAs diverge the most from host genome composition.