Subunit Interaction O F a Temperature-sensitive Alcohol Dehydrogenase Mutant in Maize1

EMPERATURE-sensitive mutations, i.e., mutations which render enzymes ‘inactive only at certain temperatures. have proven to be extremely useful in genetic analyses. Such mutations are usually detected by the inability of an organism to grow or carry out a particular function at high temperatures. In the course of ethyl methanesulfonate (EMS) niutation studies a temperature-sensitive allele of the alcohol dehydrogenase gene ( Adh,) was obtained in maize. This mutation was recognized as being temperature-sensitive in the course of physicalchemical studies on the enzymes specified by the mutant Adh, alleles. Alcohol dehydrogenase of maize behaves as a dimer. A number of electrophoretically distinguishable mutants have been described and a hybrid isozyme of intermediate mobility is formed in heterozygotes (SCHWARTZ and ENDO 1966). This investigation was initiated to study the interaction of a temperature-sensitive subunit with a wild-type (reference) subunit in heterodimers. Three possible types if interactions are. (a) positive complementation ( FINCHAM and PATEMAN 1957). rendering the heterodimer fully active at high temperature; (b) negative complementation ( FOLEY. GILES and ROBERTS 1965), resulting in the heterodimer becoming temperature-sensitive and inactive at the high temperature; and (c) noncomplementation, in which the subunits do not interact and each retains its own property in the heterodimer. A c-type heterodimer might be expected to have only half of the wild-type activity at high temperatures if only one of the two subunit; is active.