336 



VITAMIN REQUIREMENTS 



8. RIBOFLAVIN 



Riboflavin (vitamin B 2 , lactoflavin) is only rarely required by micro- 

 organisms isolated from nature. Several of the lactic acid bacteria are 

 riboflavin-deficient (192); the yeasts appear not to require it (29). A 

 single report, without data, indicates that Poria vaillantii requires ribo- 

 flavin (119); this is the first such claim for a filamentous fungus. Die- 

 tyostelium spp. have an absolute requirement (271). 



However, riboflavinless mutants can usually be isolated by appropri- 

 ate techniques (23, 77, 115, 121, 195). Of these, the most interesting is 

 the mutant of Neurospora crassa isolated by Mitchell and Houlahan 

 (172). This strain does not require exogenous riboflavin at moderate 

 temperatures, but only if grown at 28° C or higher (Figure 3). Even 

 at the elevated temperature, the mutant synthesizes riboflavin if it is 

 provided initially with some of the vitamin. 



Riboflavin has been shown directly to be formed by many yeasts and 

 fungi (60, 91, 100, 137, 172, 189, 288, 302, 335). In two yeastlike fungi, 

 Eremothecium ashbyii and Ashbya gossypii, yields are high enough so 

 that commercial use is practicable (75); it is believed that up to 6 mg 

 per milliliter of free riboflavin is produced by A. gossypii in a complex 

 medium. Although esters of riboflavin, especially flavin adenine dinu- 

 cleotide, occur in the cell, only free riboflavin is found in the medium 

 (252, 326). 



The physiological conditions of riboflavin synthesis in the yeastlike 

 fungi have been of both practical and theoretical interest. Although 

 even relatively small amounts of iron inhibit riboflavin synthesis by 

 Clostridium acetobutylicum (109) and Candida spp. (275), Ashbya 



0.25 



Figure 3. The response of a 

 Neurospora crassa mutant to 

 exogenous riboflavin. Curve 

 1, at 30°C; curve 2, at 25°. 

 Redrawn from Nicholas (180), 

 by permission of the Journal 

 of General Microbiology. 



Riboflavin, /ug per 100 ml 



