There is a single major alcohol dehydrogenase (ADH) and a single major aldehyde dehydrogenase (AldDH) in
Aspergillus nidulans
. Both ADH and AldDH are induced by ethanol and by acetaldehyde and both are subject to carbon catabolite repression. ADH and AldDH are necessary for the utilization of ethanol and of threonine, indicating that both compounds are utilized via acetaldehyde. ADH and AldDH each give a single major activity band on gel electrophoresis. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of cell extracts shows at least two similar ADH polypeptides of approximate relative molecular mass (r. m. m.) 41000 and two similar AldDH polypeptides of approximate r. m. m. 57000. The
in vitro
translation of mRNA from induced, carbon derepressed wild-type cells gives up to three ADH polypeptides in the r. m. m. range 39000-43000 and an AldDH polypeptide of approximate r. m. m. 57000. The mRNA from uninduced, carbon repressed wild-type cells does not direct the synthesis of the ADH and AldDH polypeptides. This indicates that the regulation of ADH and AldDH is at the level of transcription and/or post-transcriptional modification. The probable explanation of the multiple ADH polypeptides is post-transcriptional modification of the mRNA. Allyl alcohol mutants were made by using diepoxyoctane and γ-rays as mutagens. There are two classes,
alcA
and
alcR
. Neither class can utilize ethanol or threonine as a carbon source. The
alcA
mutants lack normal ADH and are recessive. Of the 47
alcA
mutants examined 39 do not make the ADH polypeptides while eight do so. Therefore
alcA
is the structural gene for ADH. The two
alcA
mutants tested do not make functional mRNA for ADH. The
alcR
mutants lack both ADH and AldDH and are recessive. No
alcR
mutants make the ADH or the AldDH polypeptides. The three
alcR
mutants tested do not make functional ADH or AldDH mRNA. The mutant
alcR
125 is a nonsense mutant, which establishes that
alcR
codes for a protein. The
alcA
and
alcR
genes are adjacent on chromosome VII and a preliminary fine-structure map of the
alcA
gene has been made. Three mutants that cannot utilize ethanol or threonine and have ADH, but lack AldDH, define a gene
AldA
on chromosome VIII. The
aldA
23 mutant makes the AldDH polypeptides, the other two
aldA
mutants do not. Therefore
aldA
is probably the structural gene for AldDH. Our current hypothesis is that
alcA
and
aldA
are the structural genes for ADH and AldDH respectively and
alcR
is a transacting regulatory gene coding for a protein whose function is necessary for the expression of the
alcA
and
aldA
genes.