BIOGENESIS OF THE B VITAMINS 81 



It is clear that some of the above facts have a bearing on the problem 

 of biosynthesis of B vitamins other than thiamine. 



An interpretation of the various findings is in line with the idea that 

 microorganisms — bacteria and molds — often play an important role in 

 furnishing green plants with thiamine (and probably various other B 

 vitamins) especially during the initial stages of growth of seedlings. 

 Synthesis of thiamine by green plants themselves probably can take place, 

 but in nature a complex symbiosis is the rule rather than the exception. 

 The nutritional interrelationships between the symbiotic organisms are 

 mostly obscure, and it would be unscientific to ascribe the effects of the 

 symbiosis to one organism (the green plant) merely on the basis of its 

 being more prominent visually. 



Little is known about the precise mode of thiamine biosynthesis, beyond 

 the fact that the last step involves the coupling of the pyrimidine and 

 thiazole fragments. It has been suggested, 22 - 23 > 24 however, that the thi- 

 azole fraction may be synthesized by the condensation of methionine, 

 acetaldehyde and ammonia, since similar condensations are well known: 



NH 3 CHO— CH 3 N- C— CH 3 



>■ 



CH 3 CH 2 — CH 2 — CH— COOH HC C— CH 2 — CH— COOH 



\ / NH 2 \ / NH 2 



S S 



The a-amino-/?-(4-methylthiazole-5) -propionic acid so formed may then » 

 be converted to the thiazole fraction, and it has been demonstrated that 

 yeast cells are able to perform this latter step in a manner analogous to 

 that in which they convert most a-amino acids to alcohols: 



N C— CH 3 N C— CH 3 



II II HOH || || 



HC C— CH 2 — CH— COOH > HC C— CH 2 — CH 2 OH+C0 2 + NH 3 



V k V 



Another mode of biosynthesis involving thioformamide and acetopropyl 

 alcohol has been suggested by the work cited above. 



Riboflavin 



The bacterial synthesis of riboflavin has been widely observed. Among 

 the 136 bacteria with "growth factor" requirements listed by Peterson 

 and Peterson, 1 18 synthesize riboflavin, 16 do not require it (and pre- 

 sumably synthesize it) , while 45 require it for growth, and no information 

 is available for 57. In another table the authors list 75 bacteria which 

 have been found to synthesize riboflavin. Molds and fungi are also able 

 to produce riboflavin ; one fungus Eremothecium ashbyii produces enough 

 so that it or a derivative crystallizes in the vacuoles. 25 The synthesis of 



