37. NUCLEIC ACID AND PROTEIN SYNTHESIS 373 



that the specific amino acid-binding component of sRNA be called "trans- 

 fer RNA" because of its role in transferring amino acids from activating 

 enzymes to microsomal protein. These terms — sRNA and transfer RNA, 

 the operational and the functional — are both useful and will be used here- 

 inafter where appropriate. 



It has generally been found that most of the sRNA may be almost quan- 

 titatively precipitated from the 100,000 g supernatant fraction of mamma- 

 lian tissues along with a considerable fraction of the total soluble protein — 

 by adjusting the pH to about 5. The protein component of this "pH 5 

 fraction" (earlier called the "pH 5 enzyme" 116 ) consists of the bulk of the 

 amino acid activating enzymes, as well as enzymes catalyzing terminal 

 nucleotide additions to transfer RNA, in addition to other unidentified 

 protein material. The remaining supernatant fraction (referred to as the 

 S 4 fraction 122 ) in mammalian tissues contains some amino acid activating 

 activity, terminal nucleotide addition enzymes, other enzymic components, 

 and a small amount of sRNA. The association of activating enzymes and 

 sRNA in the pH 5 precipitate is probably fortuitous, as other methods of 

 fractionation give an activating enzyme fraction free of RNA. 123 



Transfer RNA, and reactions catalyzing attachment of amino acids to 

 it, have now been described in bacterial (cf. Berg and Ofengand 13 ), 124 ' 125 

 yeast, 126 protozoan, 127 mammalian (cf. Schweet et al., m see also Hoagland 14 ), 

 and avian tissues 129 ; indeed in all tissues which have been specifically 

 examined for them. It is remarkable how good the agreement is among 

 various investigators with respect to the details of the nature of transfer 

 RNA and the reactions in which it has been shown to participate. It seems 

 highly likely, therefore, that a common mechanism of amino acid-transfer 

 RNA interaction may exist over a wide phylogenetic range. 



Transfer RNA may be obtained in good yield by treating the soluble 

 cell fraction, or the pH 5 precipitable fraction, with phenol, followed by 

 alcohol precipitation after the method of Gierer and Schramm, 24 and 

 Kirby. 130 This product usually contains less than 1 % protein, has a 260/280 



122 L. I. Hecht, P. C. Zamecnik, M. L. Stephenson, and J. F. Scott, J. Biol. Chem. 233, 

 954 (1958). 



23 E. Herbert, Ann. N. Y. Acad. Sci., 81, 679 (1959). 



24 S. Lacks and F. Gros, J. Mol. Biol. 1, 301 (1959). 



25 A. Tissieres, J. Mol. Biol., 1, 365 (1959). 



26 R. Monier, M. L. Stephenson, and P. C. Zamecnik, Biochim. et Biophys. Acta, in 

 press. 



27 J. Mager and F. Lipmann, Proc. Natl. Acad. Sci. U. S. 44, 305 (1958). 



28 R. Schweet, F. C. Bovard, E. Allen, and E. Glassman, Proc. Natl. Acad. Sci. 

 U. S. 44, 173 (1958). 



29 S. B. Weiss, G. Acs, and F. Lipmann, Proc. Natl. Acad. Sci. U. S. 44, 189 (1958). 



30 K. S. Kirby, Biochem. J. 64, 405 (1956). 



