Crystalline Thiogalactoside Transacetylase

Coordinate expression of enzymes is well established for a number of bacterial systems.Coordinate repression was first observed by Ames and Garry, who noted that addition of histidine to growing cultures of Salmonella resulted in decreased levels of four enzymes involved in the biosynthesis of histidine (1).Coordinate induction, the formation of more than one enzyme on addition of a single inducer, is known to occur in Escherichiu coli for the lactose system (2), /3-galactosidase, galactoside permease, and thiogalactoside transacetylase (3).The formation of ,&galactosidase and thiogalactoside transacetylase in whole cells has been studied in detail.It is known that an inducible (wild-type) strain produces /3-galactosidase at a rate during growth which depends on the chemical nature of the gala&side added to the medium.Thus isopropyl thiogalactoside is a highly effective inducer, whereas lactose is much less effective.When the activity of thiogalactoside transacetylase was determined, it was found that high levels of this enzyme as well were formed in response to addition of isopropyl thiogalactoside to the medium, and low levels were formed when lactose was the inducer.Another important observation has been the demonstration that constitutive mutant strains can be isolated.These mutants differ from the wild type by a single mutation and yet synthesize all components of the lactose system in the absence of inducer (2).It is evident from these findings that the control mechanism or mechanisms which govern the formation of these proteins operate on the lactose system as a group.Quantitative evaluation of coordinate expression of @-galactosidase and thiogalactoside transacetylase has been accomplished by measurements of enzyme activities.It has been considered to be necessary and important to compare concentrations as well as activities of both enzymes.The former has been purified and crystallized in several laboratories (4, 5) and the relative quantity of this protein can be related to activity measurements.A procedure for the preparation in good yield of crystalline thiogalactoside transacetylase, some analyses and characteristics of this enzyme, and a comparison of the relative quantities of the two enzymes of the lactose system are presented here.EXPERIMENTAL PROCEDURE ~Maferials-Coenzyme A was purchased from the California Corporation for Biochemical Research.Isopropyl p-n-thiogalactoside was prepared in this laboratory according to Helferich and Tiirk (6).A commercial product (Mann Research Laboratories) was recrystallized from dioxane before use.Acetyl phosphate was obtained from Worthington Biochemical Corporation.