Abstract
Background Trichomonas vaginalis has an unusually large genome (∼160 Mb) encoding ∼60,000 proteins. With the goal of beginning to understand why some Trichomonas genes are present in so many copies, we characterized here a family of ∼123 Trichomonas genes that encode transmembrane adenylyl cyclases (TMACs).Methodology/Principal FindingsThe large family of TMACs genes is the result of recent duplications of a small set of ancestral genes that appear to be unique to trichomonads. Duplicated TMAC genes are not closely associated with repetitive elements, and duplications of flanking sequences are rare. However, there is evidence for TMAC gene replacements by homologous recombination. A high percentage of TMAC genes (∼46%) are pseudogenes, as they contain stop codons and/or frame shifts, or the genes are truncated. Numerous stop codons present in the genome project G3 strain are not present in orthologous genes of two other Trichomonas strains (S1 and B7RC2). Each TMAC is composed of a series of N-terminal transmembrane helices and a single C-terminal cyclase domain that has adenylyl cyclase activity. Multiple TMAC genes are transcribed by Trichomonas cloned by limiting dilution.Conclusions/SignificanceWe conclude that one reason for the unusually large genome of Trichomonas is the presence of unstable families of genes such as those encoding TMACs that are undergoing massive gene duplication and concomitant development of pseudogenes.
Highlights
Trichomonas vaginalis, the most important sexually transmitted protist, causes vaginitis in women and urethritis in men [1,2,3]
We were interested in why some Trichomonas genes are present in so many copies and focused on one a large family of predicted transmembrane adenylyl cyclases (TMACs)
These TMACs are of particular note because (1) they have a predicted topology different from those of other metazoan and protist transmembrane cyclases, and they appear to have originated via gene duplication in Trichomonas and closely related species (e.g. Tritrichomonas and Paratrichomonas; see below) [12,13,14,15], and (2) we discovered numerous in-frame stop codons and frame shifts in these genes, Genomics of Trichomonas Transmembrane Cyclases
Summary
Trichomonas vaginalis, the most important sexually transmitted protist, causes vaginitis in women and urethritis in men [1,2,3]. Trichomonas lives under microaerophilic conditions in the lumen of the vagina by means of fermentation enzymes that are present in a modified mitochondrion called the hydrogenosome [5]. This organelle lacks enzymes of oxidative phosphorylation but makes hydrogen, and many of its fermentation enzymes were acquired from bacteria by horizontal gene transfer [6]. Recent whole genome sequencing showed an ,160-Mb Trichomonas genome encoding ,60,000 proteins [8]. This genome is bigger than those of many other medically important protists but is characteristic of trichomonads. With the goal of beginning to understand why some Trichomonas genes are present in so many copies, we characterized here a family of ,123 Trichomonas genes that encode transmembrane adenylyl cyclases (TMACs)
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