Mühlhausen, S. | bioRxiv Science

bioRxiv2020

Proteogenomics analysis of CUG codon translation in the human pathogen Candida albicans

Candida yeasts causing human infections are spread across the yeast phylum with Candida glabrata being related to Saccharomyces cerevisiae, Candida krusei grouping to Pichia spp., and Candida albicans, Candida parapsilosis and Candida tropicalis belonging to the CTG-clade. The latter lineage contains yeasts with an altered genetic code translating CUG codons as serine using a serine-tRNA with a mutated anticodon. It has been suggested that the CTG-clade CUG codons are mistranslated to a small extent as leucine due to mischarging of the serine-tRNA(CAG). The mistranslation was suggested to result in variable surface proteins explaining fast host adaptation and pathogenicity. Here, we re-assessed this potential mistranslation by high-resolution mass spectrometry-based proteogenomics of multiple CTG-clade yeasts, various C. albicans strains, isolated from colonized and from infected human body sites, and C. albicans grown in yeast and hyphal forms. Our in vivo data do not support CUG codon mistranslation by leucine. Instead, (i) CUG codons are mistranslated only to the extent of ribosomal mistranslation with no preference for specific amino acids, (ii) CUG codons are as unambiguous (or ambiguous) as the related CUU leucine and UCC serine codons, (iii) tRNA anticodon loop variation across the CTG-clade yeasts does not result in any difference of the mistranslation level, and (iv) CUG codon unambiguity is independent of C. albicans strain pathogenicity or growth form.

genomics↗

bioRxiv2020

Evidence for strong mutation bias towards, and selection against, T/U content in SARS-CoV2: implications for attenuated vaccine design.

Large-scale re-engineering of synonymous sites is a promising strategy to generate attenuated viruses for vaccines. Attenuation typically relies on de-optimisation of codon pairs and maximization of CpG dinculeotide frequencies. So as to formulate evolutionarily-informed attenuation strategies, that aim to force nucleotide usage against the estimated direction favoured by selection, here we examine available whole-genome sequences of SARS-CoV2 to infer patterns of mutation and selection on synonymous sites. Analysis of mutational profiles indicates a strong mutation bias towards T with concomitant selection against T. Accounting for dinucleotide effects reinforces this conclusion, observed TT content being a quarter of that expected under neutrality. A significantly different mutational profile at CDS sites that are not 4-fold degenerate is consistent with contemporaneous selection against T mutations more widely. Although selection against CpG dinucleotides is expected to drive synonymous site G+C content below mutational equilibrium, observed G+C content is slightly above equilibrium, possibly because of selection for higher expression. Consistent with gene-specific selection against CpG dinucleotides, we observe systematic differences of CpG content between SARS-CoV2 genes. We propose an evolutionarily informed gene-bespoke approach to attenuation that, unusually, seeks to increase usage of the already most common synonymous codons. Comparable analysis of H1N1 and Ebola finds that GC3 deviated from neutral equilibrium is not a universal feature, cautioning against generalization of results.

evolutionary biology↗