212 STERLING EMERSON 



sulfonamide-requiring strain produces something in the order of one hun- 

 dred times as much />-aminobenzoic acid as is required for essential reactions, 

 or about fifty times as much as is required for the reaction which makes 

 threonine unavailable for growth. 



Limited direct tests of nuclear frequencies in such heterocaryons indicate 

 that nuclei carrying only the sulfonamide-requiring gene are much less fre- 

 quent than those carrying the aminobenzoicless gene as well. In one test of 

 about one hundred nuclei, all proved to be double mutants. In another test, 

 conidia from heterocaryons were transferred to fresh growth tubes which 

 contained a concentration of sulfanilamide sufficient to inhibit growth of the 

 double mutant very strongly and still be favorable to the growth of the simple 

 sulfonamide-requiring mutant. Only one of five such transfers grew — again 

 suggesting that simple sulfonamide-requiring nuclei were infrequent. 



If in order to have rapid growth there must be many double mutant nuclei 

 and few simple mutants, it is not surprising that vigorous growth should 

 cease rather suddenly. Ryan, Beadle, and Tatum (1943) have shown that 

 growth substances can be transported for a distance of about one centimeter 

 in the mycelium of Neurospora. One sulfonamide-requiring nucleus at a dis- 

 tance of about a centimeter from the tip might supply enough ^-aminoben- 

 zoic acid for the growth of that tip. But as the tip grows, that nucleus might 

 easily be left behind. A deficiency of />-aminobenzoic acid would then de- 

 velop in the tip, and growth would be arrested unless a nucleus of the proper 

 constitution happened to migrate into the tip. 



Attempts to obtain rapidly growing heterocaryons involving the sulfona- 

 mide-requiring mutant and the sulfonamide-requiring, homocysteineless 

 double mutant were unsuccessful. It may be that it is impossible to have a 

 nuclear ratio which will produce sufl&cient, but not too much methionine, 

 and at the same time sufficient threonine for the requirement of the hetero- 

 caryon. 



Interpreting Suppressor Heterocaryosis Based on Model Experiments 



The heterocaryons between the sulfonamide-requiring mutant and its 

 double mutants with aminobenzoicless and homocysteineless were set up as 

 models which should duplicate the behavior observed in the sulfonamide- 

 requiring strain when suppressor mutations occurred, provided the interpre- 

 tation placed on them was correct. For this purpose, the results obtained 

 were gratifying. We should like to know just where each of the suppressor 

 mutations studied fits into the biochemical scheme, but at present it can be 

 shown only that they fit in a general way. 



Four suppressors in the first lot of six (those illustrated in Fig. 12.4), 

 which are the only ones that have been studied in any detail at all, appar- 

 ently represent mutation at four different loci, though almost no direct tests 



