MECHANISMS OF GENE ACTION 291 



thctic group attaches has been analyzed for its amino acid composition 

 and sequence; the results are summarized in Table 10.3. 



In summary, it is not yet evident to what extent the primary structure 

 of proteins uniquely determines their folded configuration, and thereby 

 their functional properties. Thus, the geneticist cannot yet be assured 

 that functional differences in enzymes extracted from wild-type and 

 mutant cells result from substitutions of one amino acid for another. 

 Nonetheless, amino acid substitution is clearly one mechanism by which 

 the properties of a protein can be altered. For this reason alone, the 

 hypothesis that mutations are expressed as altered amino acid sequences 

 is a very appealing one. Let us turn now to data on the phenomenon 

 of complementation and suppression which do not seem to fit so readily 

 into this picture. 



ALLELIC COMPLEMENTATION 



Allelic complementation consists in the formation by heterokaryons or 

 heterozvgotes made from two allelic mutants of an active enzyme which 

 neither produces alone, hi the examples so far reported, the extent of 

 enzyme activity is always far below that of the wild type. This phenom- 

 enon can be distinguished from the one discussed in Chapter 6, in which 

 pairs of mutants are classified into different cistrons by the cis-trans 

 test. There, full wild-t\'pe activity is exhibited when mutants from dif- 

 ferent cistrons are combined in the trans position. 



Complementing allelic mutants have been reported at a number of 

 loci in Neurospora, and it is beginning to look as if the phenomenon 

 were very widespread. Initially, measurements were based upon restora- 

 tion of growth but, for further analysis of the phenomenon, it is essential 

 to study the enzyme itself; growth is an indirect and qualitative index 

 of restored enzyme activity. Another important feature of complementa- 

 tion is that only a fraction of the mutants recovered are able to comple- 

 ment; this generalization is based upon reports of studies of about 15 

 different loci. 



The most extensi\'e study of complementation in relation to linkage 

 within a locus has been reported for the pan-2 region of Neurospora 

 (Figure 10.9). A large number of mutants, both complementing and 

 noncomplementing, have been mapped by recombination analysis, and 

 the two types are found interspersed along the genetic map. Whatever 

 determines ability or inability to complement is not localized in any 

 particular region within the functional unit. 



The complementing mutants can also be mapped in relation to one 



