A ModelforProtein Synthesis Involving theIntermediate -Formation ofPeptidyl-5S RNA

A modelforprotein synthesis isproposed inwhichthedonorforthepeptide elongation reaction is peptidyl-5S RNA.Space-filling modelsshowthatpeptide bondformation betweenpeptidyl-5S RNA andaminoacyl- tRNAiseminently feasible fromastereochemical pointof view.Thepeptide istransferred to5SRNA,whileatthe sametimethedeacylated tRNA isexchanged bya new aminoacyl-tRNA acceptor. Two peptidyl transferases are required bythemodel,bothofwhichhavesites forbind- ingthetermini ofbothaminoacyl-tRNA andpeptidyl-5S RNA.The modelmakesdetailed predictions aboutthe properties ofthetransferases. Ina current modelofprotein synthesis (1-4), itisassumed thatpeptide bondformation occursbytransfer ofthegrowing peptide chain frompeptidyl-tRNA toan aminoacyl-tRNA. Whilethere isno doubtthatthelatter istheacceptor inthe peptide elongation reaction, andthatthegrowing peptide remains esterified toan RNA throughout thesynthesis ofthe protein, there isinfact no evidence thatthedonorisa pep- tidyl-tRNA. Thereis, however, evidence tothecontrary. For example, itisknownthatribosomes containing active peptidyl transferase arecompletely devoid ofpeptidyl-tRNA hydrolase activity (5,6),andIhaverecently found (7) thatneither is there peptidyl-tRNA hydrolase activity intheisolated ribo-