STRUCTURAL MODEL FOR THE SELENOCYSTEINE-SPECIFIC ELONGATION-FACTOR SELB

A structural model was established for the N-terminal part of translat ion factor SelB which shares sequence similarity with EF-Tu, taking in to account the coordinates of the EF-Tu 3D structure and the consensus of SelB sequences from four bacteria. The model showed that SelB is h omologous in its N-terminal domains over all three domains of EF-Tu. T he guanine nucleotide binding site and the residues involved in GTP hy drolysis are similar to those of EF-Tu, but with some subtle differenc es possibly responsible for the higher affinity of SelB for GTP compar ed to GDP. In accordance, the EF-Tu epitopes interacting with EF-Ts ar e lacking in SelB. Information on the formation of the selenocysteyl-b inding pocket is presented. A phylogenetic comparison of the SelB doma ins homologous to EF-Tu with those from EF-Tu and initiation factor 2 indicated that SelB forms a separate class of translation factors.