Amino-acyl sRNA

Recently the biosynthesis of the amino acyl derivative of phospha-tidylglycerol, lysyl phosphatidylglycerol (cf. Fig. 15), has been reported (Lennarz et al, 1966). Using C -lysine or C -lysyl-sRNA and crude extracts of S. aureus, the reaction was found to proceed in two steps. In the first step, catalyzed by a soluble enzyme, and requiring ATP, Mg++ and sRNA, lysine is activated to form lysyl-sRNA. A particulate enzyme catalyzes the second step of the reaction, the transfer of the lysyl moiety from sRNA to endogenous phosphatidylglycerol. This appears to be the first known instance in which amino acyl sRNA is involved in the synthesis of 0-acyl esters of amino acids. 

These findings, as well as the observations of Strominger and his co-workers on the participation of glycyl-sRNA in the biosynthesis of the polyglycine side chain of the cell wall mucopeptide (see Section V, b), suggest that amino acyl sRNA derivatives may be involved in a variety of biosynthetic reactions in the cell, in addition to playing a key role in protein biosynthesis. 

At least three stages are now believed to be involved in protein synthesis in animal cells. In stage (j), amino acids (AA) are activated by the formation of amino acyl adenylates in the presence of appropriate activating enzymes (E). In stage (2), the activated amino acid reacts with ribonucleic acids of relatively low molecular weight, known as soluble (or transfer) ribonucleic acids (sRNA). In stage (j), the amino acid is transferred to the RNA of the microsomes (Ms) where it is incorporated into new protein ... 

According to this proposal, the incorporation of proline into collagen follows the acyl adenylate and acyl RNA stages now generally believed to occur in protein synthesis. A bound hydroxyproline intermediate is postulated, from which hydroxyproline is transferred to soluble RNA. We prefer to suggest that different RNA acceptor molecules exist for hydroxyproline and proline this would be consistent with recent work which indicates that the soluble RNA molecule contains the information for incorporation of a particular amino acid into protein (3). Although it is conceivable that there is hydroxylation of prolyl-sRNA to yield hydroxyprolyl-sRNA, an additional mechanism would be needed... 

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