Purification and biochemical characterization of SELB, a translation factor involved in selenoprotein synthesis

The product of the selB gene from Escherichia coli is required for co-translational insertion of selenocysteine into protein. To make the SELB protein accessible to biochemical analysis, the protein was purified from cells that overexpressed the selB gene from a phage T7 promoter plasmid. It was calculated that the overproduced SELB protein was purified 20-fold. The N-terminal amino acid sequence of the purified protein was determined, and it confirmed that the initiation codon of selB mRNA translation overlaps the stop codon of the preceding selA gene by 4 bases. Structural similarity between SELB and elongation factors was demonstrated by limited proteolysis of SELB by trypsin. The cleavage sites within SELB were identified by N-terminal sequencing of the two proteolytic products. The position in the SELB protein of the major cleavage site was homologous to a tryptic cleavage site which is characteristic for elongation factors. Immunological analysis showed that the levels of SELB are equivalent in aerobically and anaerobically grown cells; the amount of the protein was estimated to be approximately 1100 copies/E. coli cell. Upon fractionation of cell extracts, SELB was found to be partially associated with the ribosomes. The results therefore indicate that SELB is the first known elongation factor-like protein that has specificity for a particular charged tRNA.