Biochemical Genetics (II) 



291 



Evidence has been obtained that this enzyme 

 is dissociable into two portions. This be- 

 havior is reminiscent of that of tryptophan 

 synthetase, and suggests that two polypep- 

 tides, and hence two cistrons, are involved in 

 specifying this enzyme. 



It is possible, by crossing two mutant indi- 

 viduals, to obtain progeny that are hybrid for 

 separately arisen point mutants defecting 

 adenylosuccinase. If these mutants are 

 reconic alleles, and reverse mutations to 

 adenine independence are recognized and dis- 

 counted, the haploid ascospores produced by 

 the hybrid will all still require adenine in the 

 diet in order to grow. But, if the mutants 

 involve different recons, then crossovers may 

 be produced which have normal adenylo- 

 succinase activity (see Figure 32-5). 



Large numbers of ascospores need to be 

 screened to detect these recombinants, for 

 these are expected to be rare. But this can 

 be done simply by placing the ascospores on 

 minimal medium and scorinii the number 



that form growing cultures. Of all tested 

 spores, the percentage that grow, times two, 

 equals the crossover percentage, or map dis- 

 tance, between the defective recons. When 

 20 or so independently occurring point mu- 

 tants were crossed together in various combi- 

 nations, it was found that a small number did 

 not show recombination, indicating that they 

 are mutant in the same recon. But a large 

 number of these mutants proved, by showing 

 recombination, to involve different, non- 

 allelic recons, and the map distances obtained 

 between them were consistent with the hy- 

 pothesis that they are located in a linear 

 array. Moreover, their arrangement was 

 found to be continuous with that of recons 

 in different cistrons in the same chromosome. 

 These results suggest that there are more 

 recons involved in the specification of 

 adenylosuccinase than there are cistrons. 

 Accordingly, // is hypothesized that a cistron 

 is composed of a mimber of linearly arranged 

 recons. 



FIGURE 32-5. Normal {udenylosuccinase-specifying) cistron resulting from crossing 

 over within two mutant (adenylosuccinase-defective) cistrons. 



TETRAD 



y Mutant Cistron 1 

 y Mutant Cistron 2 



MEIOTIC 

 PRODUCTS 



Mutant Cistron 1 

 Double Mutant Cistron 

 Normal Cistron 

 Mutant Cistron 2 



