MANNER OF PRODUCTION OF MUTATIONS 541 



These spectra based on fungus-spore results include data for shorter wave 

 lengths than those obtained in other material, and they consistently show 

 a minor peak of efficiency which lies considerably to the left (i.e., in the 

 direction of shorter wave lengths) of the major peak, that for nucleic acid 

 absorption. This minor peak probably represents mutagenic activations 

 arising in protein. Actually, the protein absorption curve continues to 

 rise to the left for ever shorter wave lengths, rather than falling again as 

 the mutational action spectrum does, but this apparent discrepancy is 

 doubtless caused by the extremely short radiation becoming so much 

 absorbed in the superficial layers of the spores that very little of it comes 

 within effective range of the genetic material. 



Moreover, in work of McAulay and Ford (1947) on the irradiation of 

 Chaetomium spores the action spectrum for the production of most types 

 of morphological " saltants " by ultraviolet was found to correspond with a 

 part of the absorption spectrum of protein rather than with that of 

 nucleic acid. The peak efficiency was found to lie at about 2800 A (as if 

 dependent on amino acid residues having cyclic R groups) instead of at 

 about 2650 A. Although the reason for the difference between this result 

 and that on the previously cited material is not known, it must be inferred 

 either that in these spores the activations in the protein are, relatively to 

 those in the nucleic acid, more mutagenically efficient than in the other 

 material studied, for chemical reasons, or that there is relatively more 

 protein present in them in such a highly localized position as to absorb 

 much of the ultraviolet which would otherwise have reached the nucleic 

 acid and that quanta absorbed in this protein can affect (indirectly or 

 directly) the structure of the genetic material. At any rate, this adds 

 to the evidence that the mutagenic pathway of ultraviolet may follow 

 alternative routes, at least over a part of its course. 



In Chaetomium, even wave lengths longer than 3200 A, right up into 

 the visible, were found to have some slight capacity for producing mor- 

 phological saltants. The saltant-production efficiency fell oiT at 3656 and 

 4047 A to only about 3^^00 and ^ 15,000, respectively, of what it had 

 been at its peak. Yet, because of the exceedingly low damaging effect 

 it was possible to step up the energy delivered at these wave lengths so 

 much as to obtain a pronounced production of abnormal colonies. 



Although wave lengths longer than 3200 A have in most other work 

 been reported to be nonmutagenic, it has been found that they can be 

 rendered mutagenic by means of the photodynamic action of fluorescent 

 substances. Doring (1938) had obtained an apparent increase in the fre- 

 quency of mutations in Neurospora by means of visible light when the 

 organisms had been stained with eosin, but the results were of doubt- 

 ful statistical significance. Stubbe (1940) reported that visible light 

 increases the mutation rate in eosin-impregnated Antirrhinum. Kaplan 

 (1948a, 1949a, b, 1950a) ()l)tained definite evidence that, in Bacterium 



