A series of genes whose regulation is coordinated
is called an operon. Three structural
genes that code for the synthesis of lactose-degrading
enzymes (genes lacZ, lacY, and lacA)
form the lactose operon (lac operon). These
three genes are regulated by a promoter at the
5! end and are transcribed into a common
mRNA (polycistronic transcipt). Normally they
show little activity because a lac repressor inhibits
(laci) lac mRNA snythesis. The former is
the gene product of the lacI regulator gene. E.
coli can use lactose as its sole source of carbon
and energy because large amounts of !-galactosidase
can be synthesizedwithin a short time.
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