222 CONTROL MECHANISMS IN CELLULAR PROCESSES 



however, cannot be doubted, and it is from this basis that propo- 

 nents of the flavine hypothesis have proceeded. 



The demonstration that riboflavin could sensitize the in vitro pho- 

 todestruction of the auxin indoleacetic acid and its probable pre- 

 cursor, tryptophane, and evidence that a flavoprotein could also 

 participate in the so-called "indoleacetic acid (lAA) oxidase" sys- 

 tem, formed the beginning of this approach. It is now quite clear 

 that light can affect a great many substances and reactions in the 

 presence of riboflavin and related compounds. In contrast, the only 

 relevant activity that has been ascribed to carotenoids is a role in 

 the in vitro photoinactivation of "auxin-a-lactone," and the entire 

 status of auxin-a is, of course, uncertain (Galston, 1959; Went, 1956) . 

 Thus, the flavine hypothesis has the general advantage that it is 

 easy enough to see what flavines might do, and not so easy to see 

 this for carotenoids. It is probably this, more than any other con- 

 sideration, that has led a number of recent workers to favor the 

 flavines. There are two general grounds, however, on which this 

 line of reasoning can be criticized. 



The first is that it is based primarily on in vitro experiments. 

 This is true both of the riboflavin-mediated photodestruction of 

 various compounds and of any hypothesis invoking the lAA oxidase 

 system. There is no unambiguous evidence, at least in my opinion, 

 that such events or systems operate in intact tissues at all, and there 

 are some reasons to doubt it. A second major difficulty is that the 

 flavine hypothesis provides an embarrassment of riches. At least 

 in vitro, flavines in the presence of light can bring about a great 

 many different effects. In addition, light absorption may well affect 

 the properties of flavine compounds involved in electron transfer 

 systems ( Commoner and Lippincott, 1958 ) . The question then be- 

 comes one of how to obtain any specificity from such action, and the 

 same can be asked about other yellow compounds besides flavines. 



The answer to the question of specificity may well come from 

 understanding the relationship of the photoreceptor molecule, what- 

 ever its identity, to the fine structure of other molecules and macro- 

 molecules around it, a problem about which we know almost 

 nothing as yet. In this connection, one should avoid a misleading 

 impression that might result from the exclusion of vision and pho- 

 tosynthesis from this discussion. The photoreceptor molecules for 

 these processes occur in highly ordered structures, while the loca- 

 tions of the photoreceptor molecules in other light-dependent proc- 



