Stop codon readthrough in Trichomonas is a mechanism for gene expression regulation and expanding protein function

Trichomonas vaginalis is the causative agent of trichomoniasis, a common sexually transmitted infection among women of reproductive and peri-menopausal age. The parasite has an unusually large genome, rich in complex repeats, including a vast repertoire of transposable elements and multi-copy gene families. Since very few T. vaginalis genes have introns, gene expression is usually straightforward, with ribosomal translational machinery proceeding from a start codon to the next in-frame stop codon of an unspliced poly(A)denylated mRNA. However, our previous studies raised the possibility of T. vaginalis gene expression involving stop codon readthrough (SCR), where transcription through in-frame stop codons produces longer-than-predicted mRNAs that translate to fully functional proteins. Here, we leverage long-read RNA-seq and new chromosome-scale assemblies of two T. vaginalis strains and two avian sister species to investigate and characterize ~1,400 long, mature mRNAs that contain more than one predicted protein-coding gene transcribed from what we call '' RT genes '', composites of adjacent predicted genes. We first identify RT genes in a second T. vaginalis strain and in close relatives T. vaginalis-like and T. stableri, indicating that this phenomenon is conserved among Trichomonas species and strains. Second, we find transcripts of RT genes to be more abundant by many orders of magnitude than monocistronic genes. Third, we found the distance between predicted genes within RT genes to be significantly shorter than between adjacent independent predicted genes. Fourth, functional annotation revealed that RT genes encode at least 50 distinct protein functions, suggesting that this unusual transcriptional mechanism has a role in an array of biological processes in Trichomonas. Our results from two Trichomonas species suggest that SCR is an important mechanism controlling gene expression and the diversity of protein function in this parasite.