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Dimethyl sulfoxide inhibits horse liver alcohol dehydrogenase. In the direction of aldehyde reduction, this inhibition is competitive with aldehyde, with an inhibition constant of 5 x 10(-3)M. Dimethyl sulfoxide reacts with the binary complex consisting of enzyme and the reduced form of nicotinamide -adenine dinucleotide to form a highly fluorescent ternary complex, with a dissociation constant similar to the inhibition constant. The inhibition of aldehyde reduction can be interpreted as due to competition between aldehyde and dimethyl sulfoxide for the carbonyl binding site of the above-mentioned binary complex.

The repressor of the galactose operon of Escherichia coli has been partially purified and identified as a protein. Induction of a lysogen in which lambda was linked to the bacterial galR and lysine genes resulted in a large increase in the production of the gal repressor. Single-step purification by affinity chromatography, using the ligand p-aminophenyl-beta-D-thiogalactoside linked to beaded agarose, provided a convenient method of separating the gal repressor from other DNA-binding proteins. Binding of gal repressor to lambdapgal[(32)P]DNA was studied by assay of binding to a nitrocellulose filter. Interaction between gal repressor and lambdapgal DNA showed a high degree of specificity; the dissociation constant of the complex was estimated to be 1.0 x 10(-12) M. Unlabeled lambdapgal DNA competed for binding to gal repressor, but lambdaDNA and lambdah80dlac DNA did not. Fucose and galactose, which function as inducers of the galactose operon in vivo, produced one-half maximal inhibition of gal repressor-lambdapgal DNA binding at concentrations of 5 x 10(-5) M.

A cyclic AMP binding protein has been purified over 100-fold from E. coli extracts. Protein purified from wild-type strains binds cyclic AMP with an apparent dissociation constant of 1-2 x 10(-6)M. Two mutant strains that are unresponsive to exogenous cyclic AMP have altered binding activity; the protein purified from one of these mutants has a decreased affinity for cyclic AMP (apparent dissociation constant = 2 x 10(-5)M). Extracts of this mutant are deficient in their ability to support beta-galactosidase synthesis in vitro. The addition of purified, wild-type binding protein to these extracts restores enzyme synthesis toward normal. Because this binding protein appears to be required for cyclic AMP action, we suggest it be called the cyclic AMP receptor protein (CR protein).

Both cyclic AMP and a specific inducer acting in concert are required for the synthesis of many inducible enzymes in E. coli. Little enzyme is made in the absence of either. In contrast to the specific inducers which stimulate the synthesis only of the proteins required for their metabolism, cyclic AMP controls the synthesis of many proteins. Glucose and certain other carbohydrates decrease the differential rate of synthesis of inducible enzymes by lowering cyclic AMP concentrations. In the lac operon, cyclic AMP acts at the promoter site to facilitate initiation of transcription. This action requires another protein, the cyclic AMP receptor protein. The nucleotide stimulates tryptophanase synthesis at a translational level. The action of cyclic AMP in E. coli may serve as a model to understand its action on transcriptional and translational processes in eukaryotes.