Genomic investigation of the system for selenocysteine incorporation in the bacterial domain.

The elucidation of nearly 100 bacterial genomes has made it possible to categorize them into two groups, according to the presence or absence of a selenocysteine (Sec) tRNA. In the group with the tRNA, a Sec incorporation system like that of Escherichia coli would be expected. However, for the other group, the following question has been left unsolved. Is it reasonable to assume that bacteria without the tRNA lack the entire Sec system, and do such bacteria exist commonly? To explore it experimentally, we chose Bacillus subtilis, a representative eubacterium for which a Sec tRNA has not been found. First, we reviewed the genome to search for the tRNA gene. Second, we examined the possible expression of an unknown tRNA. Third, we examined Sec-related enzymes and proteins in B. subtilis cell extracts. Fourth, the B. subtilis and E. coli seryl-tRNA synthetases were expressed, and the specificity was analyzed. Consequently, all of the data showed negative results about the existence of the Sec system in B. subtilis. Additionally, we discuss the possibility of predicting the existence or absence of the system in each bacterial organism by using the Sec tRNA and seryl-tRNA synthetase as indicators.