Thiogalactoside transacetylase. Physical and chemical studies of subunit structure.

The subunit size of thiogalactoside transacetylase from Escherichia coli ML 308 in 6 M guanidinium chloride measured by sedimentation equilibrium in the ultracentrifuge was found to be 29,700 =t 2,000, about half the molecular weight of the native protein. The two chains were identical by estimation of the number of peptides obtained after treatment with trypsin, BrCN, and with both reagents in succession. Analysis of NHz-terminal amino acids of the peptides produced by BrCN cleavage gave results which agreed with this conclusion, after suitable correction for considerable random cleavage in the solvent, 70% formic acid. The relative molar quantities of two protein products of the lac operon, P-galactosidase and thiogalactoside transacetylase, when calculated in terms of the minimum molecular weights, 135,000 and 32,000, respectively, are normally in the range of 5 to ‘7: 1 for E. coli ML and slightly less for E. coli K-12.