120 CARBON METABOLISM II 



actinom\cetes, e.g.. tlie enzyme system of Strepto?7i\ces olbus (134. 

 ict on cell wall polysaccharides and can therefore tentatively be 

 classed with the mucopolysaccharases. Enzymes of this type have been 

 reported frequentlv from actinomycetes (114a. 249) and must be 

 present in those fungi in which the chitin breaks down during 

 autolysis (216). 



I i tain marine actinomvcetes liquefv agar (107, 219). The enzyme 

 responsible has been called gelase: it is evidently either rare or absent 

 in terrestrial fungi and actinomycetes. since agar is used so generalh 

 in culture media that gelase-producing organisms would have long 

 since been detected if they were common. 



The Synthesis of Polysaccharides. The known fungal polysac- 

 charides are reviewed elsewhere (Chapter 2); it will be recalled that 

 they include chitin. polyhexoses, and more complex polysaccharides. 

 The most common hexose component is glucose, the next most com- 

 mon galactose: since all of the galactans are formed in media with 

 glucose or sucrose as carbon source, it seems probable that a galacto- 

 waldenase is present, as in yeast, catalyzing the conversion of glucose 

 to galactose (124). 



The metabolic origin of the polysaccharides of fungi has been little 

 investigated, but may be expected to follow the pathways known in 

 other organisms. The two best known mechanisms of polysaccharide 

 synthesis in bacteria are. in outline: 



CuHsA, — (CHtoOs). - (n)C6H 12 (7) 



Disaccharide Polysaccharide Monosaccharide 



(n)Glucose-l -phosphate ±^ Polysaccharide + phosphate (8) 



The first of the>e is a transghcosidation. of which the action of 

 levansucrase is an example: the second is catalyzed by a phosphorylase 

 and is exemplified b\ glycogen formation in the animal. Levan 

 s\nthesis in Aspergillus sydowi spores may be a transghcosidation; it 

 can be effectuated by a broken-cell preparation, and the levan is 

 formed during growth on sucrose but not during growth on monosac- 

 charides _ The polyglucose of Torulopsis rotundata, on the 

 other hand, is probably synthesized b\ a phosphorylase (139). Evidence 

 on polysaccharide biogenesis in the fungi is so far limited to these two 



The polysaccharides of some fungi are formed more abundantly in 

 acid than in neutral media (27, 140. 207;. but the reverse is true of 



lerotiose formation (1). In yeast, inhibition of polysaccharide 

 formation by alkalinity is believed to result from an inhibition of 



