Progress in Nucleic Acid Research and Molecular Biology, by Waldo E. Cohn (Ed.)

By Waldo E. Cohn (Ed.)

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Coli, yeast Yeast Yeast Yeast E. coli Yeast E. coli Yeast Yeast Yeast Yeast Rabbit liver Yeast Yeast E. , for yeast tRNAPhe,n = 76. Adenosine-5'-(a-thio)triphosphateexists in two diastereoisomeric forms, A and B (143). - + - + - - 134,137 118,120,134 138,139 125,138,139 112 112 112,134 134,141 112,134 134,141 142 50,143 143 112 149 116,150 112 112,128,135 148 55 112 151 112 112 20 MATHIAS SPRINZL AND FRIEDRICH CRAMER phosphate (dAMP or dA) was not incorporated into tRNA by enzymes from E . coli (132-134), rabbit liver (128),and rabbit muscle (135),but incorporation of it into yeast tRNA-N-C-C by the yeast enzyme was later successfully accomplished (120, 134, 136, 137).

Most recently, a stepwise synthesis of oligonucleotides by T4 pol ynucleotide ligase was reported (107). The low specificity of this latter enzyme with respect to the donor nucleotide may allow incorporation of various nucleotides into the 3' end of tRNA. I n all cases it is necessary to use a shortened tRNA with a well characterized and unique 3' terminus in order to obtain a modified tRNA species in which a single modified nucleotide is placed in a defined position of the -N-C-C-A end. T h e shortened tRNA can be achieved by a limited enzymic degradation of the -N-C-C-A by a 3'exonuclease, such as snake venom phosphodiesterase.

Although the determination of the site of aminoacylation using “amino” tRNAs (125, 138) is more complicated than that using the isomeric “deoxy” species for the above mentioned reasons, the nonisomerizable products of aminoacylation of tRNA-N-C-C-A(2‘NH2) or tRNA-N-C-C-A(3‘NH2)(Fig. 7d,e) are very usefuI for the study of 2‘ vs. 3‘ specificity during the other steps of protein biosynthesis (125, 140,172). The site of enzymic acylation of “oxidized-and-reduced” tRNA, tRNA-N-C-C-Aoxi-ped, has been investigated in detail by Ofengand et al.

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