Bioinformatic Prediction of an tRNA(Sec) Gene Nested inside an Elongation Factor SelB Gene in Alphaproteobacteria

In bacteria, selenocysteine (Sec) is incorporated into proteins via the recoding of a particular codon, the UGA stop codon in most cases. Sec-tRNA(Sec) is delivered to the ribosome by the Sec-dedicated elongation factor SelB that also recognizes a Sec-insertion sequence element following the codon on the mRNA. Since the excess of SelB may lead to sequestration of Sec-tRNA(Sec) under selenium deficiency or oxidative stress, the expression levels of SelB and tRNA(Sec) should be regulated. In this bioinformatic study, I analyzed the Rhizobiales SelB species because they were annotated to have a non-canonical C-terminal extension. I found that the open reading frame (ORF) of diverse Alphaproteobacteria selB genes includes an entire tRNA(Sec) sequence (selC) and overlaps with the start codon of the downstream ORF. A remnant tRNA(Sec) sequence was found in the Sinorhizobium meliloti selB genes whose products have a shorter C-terminal extension. Similar overlapping traits were found in Gammaproteobacteria and Nitrospirae. I hypothesized that once the tRNA(Sec) moiety is folded and processed, the expression of the full-length SelB may be repressed. This is the first report on a nested tRNA gene inside a protein ORF in bacteria.