1()0 



CARBON METABOLISM III 



Figure 5. Growth, fat forma- 

 tion, and carotenogenesis in 

 Pliycomyces blakesleeanus. Re- 

 drawn from Goodwin and Will- 

 mer (238), by permission of the 

 Cambridge University Press. 



Time, days 



proportions of the different carotenoids formed (215), as it does in 

 Rhodotorula rubra (392). Oxygen is required for carotene formation 

 by Neurospora crassa (572), and presumably this is true of other 

 fungi. 



The mycelial carotenoids of Neurospora crassa do not develop in 

 complete darkness, although the conidial carotenoids do; this problem 

 has been studied by Zalokar (572, 573). Very brief exposure to light 

 is sufficient to activate pigment synthesis, which is proportional to 

 light intensity up to a saturation value. The action spectrum of light 

 activation is consistent with, but does not, of course, prove the 

 participation of a flavoprotein as the photoreceptor. Calculated 

 quantum yields are so high as to make it necessary to postulate that 

 activation is an indirect process. Carotenogenesis in other fungi is 

 stimulated by light but does not require it (131, 219). Synthesis by 

 green plants does not require light. 



Carotenoid formation is inhibited by streptomycin, chloram- 

 phenicol, and isonicotinic hydrazide at concentrations which do not 

 inhibit growth (235). Diphenylamine inhibits carotenogenesis in both 

 bacteria and fungi (233, 528). Its effect is to reduce the absolute and 

 relative amounts of the more unsaturated carotenoids, e.g., /3-carotene, 

 and to increase the relative amounts of the more saturated, e.g., 

 phytofluene and neurosporene. 



The biosynthesis of the carotenoids may be considered under three 



