The mycobacterial selenocysteine machinery: presence and expression

The Mycobacterium genus includes more than 190 species that occupies diverse ecological niches. Some are non-pathogenic and environmental, whereas others cause severe diseases both in humans and animals, e.g. tuberculosis (TB) and leprosy. Selenocysteine (Sec) is present in all three domains of life. Here we report the presence of the Sec-machinery (selA, selB, selD and tRNA^Sec) genes and selenoprotein formate dehydrogenase (FDH) genes in roughly 40% of 244 mycobacterial genomes. Their presence is distributed evenly among slow and rapid-growing mycobacteria and our data indicate that they were acquired through horizontal gene transfer. Some mycobacteria however lost these genes during the evolution of the genus. We provide RNA-Seq data showing transcript levels of the Sec-machinery and FDH genes in different mycobacteria grown under different conditions. Finally, we suggest that the tRNA^Sec gene (selC), positioned immediately upstream of selA-selB, is involved in the regulation of the expression of selA-selB. Together our data expand our understanding of selenocysteine metabolism and its evolution within the Mycobacterium genus.