Role of the 1-72 base pair in tRNAs for the activity ofEscherichia colipeptidyl-tRNA hydrolase

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Abstract
Previous work by Schulman and Pelka (1975) J. Blol. Chem. 250, 542 - 547, indicated that the absence of a pairing between the bases 1 and 72 in initiator tRNAMetf, explained the relatively small activity of peptidyl-tRNAMetf hydrolase towards W-acetyl-methionyl tRNAMetf. In the present study, the structural requirements for the sensitivity of an W-acetyl aminoacyMRNA to Escherichia coll peptidyl-tRNA hydrolase activity have been further Investigated. Ten derivatives of tRNAMetf, with various combinations of bases at positions 1 and 72 In the acceptor stem have been produced, amlnoacylated and chemically acetylated. The release of the aminoacyl moiety from these tRNA derivatives was assayed in the presence of peptidyl-tRNA hydrolase purified from an overproducing strain. tRNAMetf, derivatives with either C1A72:, C1C72, U1G72, U1C72 or A1C72 behaved as poor substrates of the enzyme, as compared to those with C1G72, U1A72, G1C72, A1U72 or G1U72. With the exception of U1G72, It could be therefore concluded that the relative resistance of tRNAMetf, to peptidyl tRNA hydrolase did not depend on a particular combination of nucleotldes at positions 1 and 72, but rather reflected the absence of a base pairing at these positions. In a second series of experiments, the impairing of the 1 and 72 bases, created with C-A or A-C bases, instead of G-C in methionyl-tRNAMetf or in valyl-tRNAMetf, was shown to markedly decrease the rate of hydrolysis catalysed by peptidyl-tRNA hydrolase. Altogether, the data indicate that the stability of the 1-72 pair governs the degree of sensitivity of a peptidyl-tRNA to peptidyl-tRNA hydrolase.

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