348 GEORGE W. CROSBIE 



C 14 was due to hydrolysis to carbamyl-/3-alanine followed by carbamyl trans- 

 fer to aspartic acid must be considered. 



The possible involvement of /3-alanine in pyrimidine synthesis in the 

 pyrimidine-less mutant N. crassa 1298 has been proposed by Boyd and 

 Fairley. 173 The mutant is capable of growth on propionic acid or a-amino- 

 butyric acid. The incorporation of these precursors into the pyrimidine 

 ring has been stated without evidence to involve a derivative of /3-alanine. 

 It should be noted that certain mutants of E. coli capable of growth on di- 

 hydrouracil have recently been shown to be pantothenate-less mutants. 174 



The most significant indication of the role of dihydropyrimidines in nu- 

 cleic acid synthesis derives from the recent work of Mokrasch and Grisolia 175 

 who have reported that a soluble enzyme system of avian liver is capable 

 of incorporating carbamyl-/3-alanine, carbamyl-/3-alanine ribonucleoside, 

 carbamyl-/3-alanine 5'-ribonucleotide, dihydrouracil, dihydrouridine, and 

 dihydrouridine-5'-phosphate into the RNA of the system by a pathway 

 not involving orotic acid. The more extensive incorporation of the ribo- 

 nucleotides as compared with the other substrates strongly suggests that 

 the direct reaction sequence involves the ribonucleotide derivatives. The 

 further investigation of these reactions together with an evaluation of their 

 quantitative significance will be awaited with interest. It may be noted 

 that Visser et a/. 176 have detected negligible incorporation of dihydrouri- 

 dine-3'(2'-)-phosphate-G-C 14 into the polynucleotides of rat intestinal 

 mucosa and regenerating liver. 



A comprehensive review of pyrimidine nucleotide biosynthesis by Reich- 

 ard has appeared recently. 177 



173 M. Boyd and J. L. Fairley, Federation Proc. 17, 193 (1958). 



174 I. J. Slotnick and H. Weinfeld, J. Bacteriol. 74, 122 (1957). 



175 L. C. Mokrasch and S. Grisolia, Biochim. et Biophys. Acta 27, 226 (1958). 



176 D. W. Visser, D. Van Praag, and T. K. Fukuhara, Arch. Biochem. Biophys. 70, 

 217 (1957). 



177 P. Reichard, Advances in Enzymol. 21, 263 (1959). 



