Thiogalactoside transacetylase

Figure 39-2. The positional relationships of the structural and regulatory genes of the lac operon. lacZ encodes 3-galactosidase,/ocT encodes a permease, and lacA encodes a thiogalactoside transacetylase. lad encodes the lac operon repressor protein.

The A gene, which encodes a thiogalactoside transacetylase enzyme that is not essential for lactose metaboUsm and whose function is uncertain... 

The lactose (toe) operon (Fig. 28-7a) includes the genes for jS-galactosidase (Z), galactoside permease (F), and thiogalactoside transacetylase (A). The last of these enzymes appears to modify toxic galactosides to facilitate their removal from the cell. Each of the three genes is preceded by a ribosome binding site (not shown in Fig. 28-7) that independently directs the translation... 

In a cell that is lacZ, what would be the relative thiogalactoside transacetylase concentration, compared with wild type, under the following conditions ... 

Figure 8-27. Cross-reacting material (bars) and thiogalactoside transacetylase activity (filled circles) in extracts of nonsense mutants. A genetic map of the lac Z gene, indicating the locations of some of the mutations used in this experiment, is shown below. [From A. V. Fowler and I. Zabin, J. Mol. Biol., 33 35 (1968).]...

A crucial clue to the mechanism of gene regulation was the observation that two other proteins are synthesized in concert with P-galactosidase—namely, galactoside permease and thiogalactoside transacetylase. The permease is required for the transport of lactose across the bacterial cell membrane. The transacetylase is not essential for lactose metabolism but appears to play a role in the detoxification of compounds that also may be transported by the permease. Thus, the... 

NAs are common in bacteria. The polycistronic lac mRNA can be translated to produce b-galactosidase (LacZ), permease (LacY), and thiogalactoside transacetylase (LacA). 

Operon composition/induction - The lactose operon consists of three linked structural genes that encode enzymes of lactose utilization, plus adjacent regulatory sites. The three structural genes—z, y, and a—encode / -galactosidase, / -galactoside permease (a transport protein), and thiogalactoside transacetylase (an enzyme of still unknown metabolic function), respectively. 

Explain the stoichiometric relationship among the concentrations of (3-galactosidase, galactoside permease, and thiogalactoside transacetylase in E. coli. 

The a gene codes for thiogalactoside transacetylase. Unlike the situation with the other structural genes of the operon, a mutations do not affect the ability of the bacteria to grow on lactose. 

Transacetylase is known to catalyze the transfer of an acetyl group from acetyl CoA to a thioglalactoside to form an acetylthiogalactoside [89]. The enzyme is assayed by spectrophotometrically measuring the appearance of free coenzyme A [see (Eq. 2), which describes the action of thiogalactoside transacetylase] [90] or alternatively by quantitating... 

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