210 STERLING EMERSON 



nine deficiency can be prevented by genetic means. The simplest is of course 

 by introducing the wild type allele of the sulfonamide-requiring gene, but the 

 other two are of more interest. One of these is by introducing a genetic block 

 to the synthesis of homocysteine. Mutant strain H-98 blocks the terminal 

 step in the synthesis of homocysteine. In the double mutant — sulfonamide- 

 requiring, homocysteineless — there is no interference with the availability of 

 threonine for growth, since the deleterious reaction does not take place in 

 the absence of homocysteine. In the absence of homocysteine, however, there 

 can be no synthesis of methionine, so that the double mutant fails to grow 

 because of a methionine deficiency. The double mutant will grow if supplied 

 with exactly the right amount of methionine — more inhibits growth, because 

 methionine is degraded to homocysteine which then supports the deleterious 

 reaction (Zalokar, 1950). 



The remaining method is to introduce a genetic block to the synthesis of 

 /»-aminobenzoic acid. In the double mutant — sulfonamide-requiring, amino- 

 benzoicless — there is again no interference with the utilization of threonine 

 since there is no />-aminobenzoic acid to catalyse the deleterious reaction. 

 There is again a deficiency for methionine, because />-aminobenzoic acid is 

 needed in its synthesis. There is also a deficiency of />-aminobenzoic acid for 

 other essential processes. The double mutant will grow if supplied just the 

 right amount of /(-aminobenzoic acid to satisfy the essential requirements, 

 but not enough to stimulate the deleterious reaction (Zalokar, 1948). 



Model Heterocaryons 



It can be seen that the simple sulfonamide-requiring mutant on the one 

 hand, and the two double mutants on the other, have different deficiencies. 

 One produces methionine and ^-aminobenzoic acid, but not enough threo- 

 nine. The others produce sufficient threonine, but no methionine, and in one 

 case, no ^-aminobenzoic acid. In heterocaryons between the simple and 

 double mutants, the two types of nuclei should complement each other in 

 the production of essential growth substances. If the nuclear ratios can be so 

 adjusted that the different substances are produced in appropriate amounts, 

 vigorous growth should result. Heterocaryons involving the simple sulfona- 

 mide-requiring mutant and the double mutant sulfonamide-requiring, amino- 

 benzoicless have resulted in vigorous growth (Emerson, 1948) in every test 

 so far made. Growth curves of some of these heterocaryons are illustrated 

 in Figure 12.6. 



Growth of these heterocaryons is usually not maintained at a constant 

 rate. Growth may stop completely after a time, or it may nearly stop and 

 then start again. This is believed to be due to fluctuations in the ratio of the 

 two kinds of nuclei in the advancing hyphal tips. Apparently there must be 

 many times as many double mutant nuclei as simple sulfonamide-requiring 

 nuclei to result in a favorable combination. This is not surprising since the 



