Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 493

(a)

Promoter Operator Gene 1 Gene 2 Gene 3

RNA
polymerase

Transcription blocked

Repressor

(b)

Promoter Operator Gene 1 Gene 2 Gene 3

RNA
polymerase

Transcription permitted

Inducer Repressor

FIGURE 10.13 Process of enzyme induction. (a) A repressor protein binds to the operator region and blocks the
action of RNA polymerase. (b) Inducer molecule binds to the repressor and inactivates it. Transcription by RNA
polymerase occurs and an mRNA for that operon is formed.

a single promoter e operator is called an operon. The operon concept is central to under-
standing microbial regulation. Control can be even more complex than indicated in Figs
10.12 and 10.13 . Escherichia coli is well-studied species in terms of the promoter e operon coor-
dination. The lactose (or lac) operon controls the synthesis of three proteins involved in
lactose utilization as a carbon and energy source in E. coli. These genes are lac z (gene 1),
lac y (gene 2), and lac a (gene 3). The repressor is made on a separate gene called lac i.
Lac z encodes b -galactosidase (or lactase), which cleaves lactose to glucose and galactose.
The lac y protein is lactose permease, which acts to increase the rate of uptake of lactose
into the cell. Lac a is for thiogalactoside transacylase. Lactose is modified in the cell to allolac-
tose, which acts as the inducer. The conversion of lactose to allolactose is through a secondary
activity of the enzyme b -galactosidase. Repression of transcription in uninduced cells is
incomplete, and a low level (basal level) of proteins from the operon is made. Allolactose
acts as indicated in Fig. 10.13 , but induction by allolactose is not both necessary and sufficient
for maximum transcription. Further regulation is exerted through catabolite repression (also
called the glucose effect).
The three genes coding for enzymes necessary for lactose metabolism in E. coli are coordi-
nated in a so-called operon, and gene expression is coordinately controlled by two regulatory
sites positioned upstream of the genes (see Fig. 10.13 ): (1) Control at the operator by
a repressor protein; (2) Carbon catabolite repression at the promotor. The repressor protein
E R has two binding sites d one site that specifically ensures binding to the operator (E O )
and one site which may bind lactose (S L ). When lactose binds to the repressor protein, its
conformation changes so that its affinity for binding to the operator is significantly reduced.
Thus, lactose prevents the repressor protein from binding to the operator, and transcription
of the genes by RNA polymerase is therefore allowed. Consequently, lactose serves as an
inducer of transcription; i.e. expression of the three genes lacZ, lacY, and lacA is not possible
unless lactose or another inducer, e.g. isopropyl-13-D-thiogalactoside, abbreviated IPTG, is
present. The binding of the repressor protein to lactose and the operator may be described by

k 1

E R þ 4 S L % E R $ð S L Þ 4 (10.5)

k 1

k 2
k 2

E O þ E R % E O $ E R (10.6)