Elongation factors EF-Tu,members of the family of GTP-binding proteins,belong to key elements of protein synthetic machinery in all bacteria.They are responsible for the delivery of amino acids,building elements of proteins,to mRNA-translating ribosomes,where they are incorporated into growing polypeptide chain.All amino acids are delivered to ribosomes by EF-Tu in the form of aminoacyl-tRNAs (aa-tRNAs).To attach aa-tRNA,EF-Tu must first bind GTP,whereas the release of aa-tRNA from EF-Tu to the A-site of ribosome during mRNA decoding requires EF-Tu-catalyzed hydrolysis of the bound GTP into GDP.This is how guanine nucleotides regulate activity of EF-Tu proteins.All examined EF-Tus are arranged into three domains:N-terminal domain 1,middle domain 2 and C-terminal domain 3.The domain 1 is also called a G-domain or a catalytic domain because it harbors the binding site for GTP/GDP and the catalytic center for hydrolysis of bound GTP.No catalytic activity has been attributed yet to domains 2+3; they are called non-catalytic domains.