402 B. MAGASANIK 



PNA was also studied by Marshak^^ by a method in which PNA was 

 hydrolyzed to a mixture of bases with hot perchloric acid/" This procedure 

 was later found to lead to an incomplete hydrolysis of the pyrimidine 

 nucleotides and to poor recoveries of cytosine and uracil.^* The use of this 

 method may account in part for the exceptionally low cytosine and uracil 

 (;ontent of nuclear PNA reported by Marshak.^^ 



4. PNA FROM Microbial Tissues 



The composition of yeast PNA has been determined in many labora- 

 tories. Most investigators who were intent upon developing methods for 

 the estimation of the nucleotide composition of PNA used commercial 

 yeast nucleic acid, purified to various degrees, as an object for testing 

 their method. However, it has been mentioned earlier that all commercial 

 yeast PNA preparations have been isolated by extraction with alkali, and 

 consequently are badly degraded. The composition of such preparations 

 cannot be considered to represent yeast PNA. For this reason only the 

 composition of yeast nucleic acid specimens isolated in the laboratory by 

 mild procedures will be considered here.'*^^^'^° The composition of five 

 preparations of PNA from baker's yeast are shown in Table IX. In all 

 cases the yeast was dried with ethanol before extraction of PNA. It can 

 be seen that these five preparations are quite similar in composition, al- 

 though the method of isolation was different, except for preparations 4 

 and 5. Preparation 6, which was isolated from brewer's yeast seems to be 

 of slightly different composition."*^ The composition of yeast PNA ap- 

 proaches a "statistical tetranucleotide" more closely than that of animal 

 PNA. However, baker's yeast PNA was in all cases found to be somewhat 

 richer in guanine than in adenine, and somewhat richer in uracil than in 

 cytosine. It must be kept in mind that all purified preparations of yeast 

 PNA account for only a portion of the PNA originally present in yeast. 

 Therefore the good agreement in the results obtained may simply indicate 

 that the same portion of yeast PNA is isolated in the procedures used. A 

 different procedure of drying, such as substituting acetone for ethanol, led 

 to a PNA preparation with a guanine content of 14.3, almost one-third 

 higher than that of the ethanol-dried preparations.^^ An attempt was made 

 by Khouvine and her collaborators to investigate the heterogeneity of 

 baker's yeast PNA by separating the PNA-proteins isolated from acetone- 

 ground yeast into several fractions by isoelectric precipitations.-^ The 

 nucleotide composition of the fractions precipitated at pH 5 and 4.3 closely 

 resembled that of the preparations presented in Table IX, whereas the 

 nucleotide composition of the fraction precipitated at a pH of 2.3 was 

 richer in giianylic acid. 



" A. Marshak and H. J. Vogel, J. Biol. Chem. 189, 597 (1951). 



