no PHYSIOLOGY OF THE FUNGI 



amides, and alanine. It may be assumed that the amides of gkitamic and 

 aspartic acid function in yeast as nitrogen carriers, as they do in green 

 plants. 



Preformed amino acids are probably used in protein synthesis. In 

 principle this process is the reverse of hydrolysis. Many complex chem- 

 ical reactions are involved. Proteins vary in complexity, the simplest 

 having molecular weights in the neighborhood of 16,000 to 17,000. The 

 molecular weight of some proteins is said to be greater than 1,000,000, 

 and tobacco mosaic virus protein is estimated to have a molecular weight 

 of 40,000,000. In spite of these enormous molecular weights, a good 

 deal is known about the structure of proteins. Fundamentally, a pro- 

 tein consists of amino-acid residues joined together by peptide linkages, 

 — CH2 — NH — CO — . Since different proteins have highly specific 

 properties which depend upon the molecular structure, the synthesis of 

 these compounds involves a systematic linking together of amino-acid 

 residues in a definite pattern. For reviews of protein structure the reader 

 is referred to Bull (1941) and Astbu^y (1943). 



The general pathways of nitrogen utilization by fungi are shown in 

 scheme III. 



Scheme III. Possible Pathways of Protein Synthesis from Various Sources 



OF Nitrogen 

 Nitrates — ■ > Ammonia > Primary amino acids 



Secondary amino acids 

 Ammonia — 



i 

 Secondary amino acids 



Primary amino acids- 



Peptides 



i 

 Polypeptides 



i. 

 Protems 



OTHER NONMETALLIC ELEMENTS 



It is not known whether fungi require nonmetallic elements other than 

 hydrogen, oxygen, sulfur, phosphorus, and nitrogen. Boron and iodine 

 are frequently added to culture media, but good evidence of their essen- 

 tiality for fungi appears to be lacking. Sodium chloride is frequently 

 added to media, but neither sodium nor chlorine, so far as is known, is 

 essential for the fungi. 



In nature fungi come in contact with many nonessential elements. 

 Some of these may be metabolized. Others may modify the life processes 

 of the fungi by their toxic action or by other means. Chlorine is found in 

 various compounds synthesized by fungi, e.g., non-ionic chlorine is found 

 in chloramphenicol, one of the newer antibiotics. Many species of fungi 

 metabolize arsenic. Penicillium hrevicaule, among other species, pro- 



