THE LAC OPERON
One strategy bacteria employ to control the expression of their genes: by
grouping functionally related genes together so they can be regulated together
easily. Such a group of contiguous, coordinately controlled genes is called an
operon.
The first operon to be discovered has become the prime example of the
operon concept. It contains three genes that code for the proteins that allow
E. coli cells to use the sugar lactose, hence the name lac operon. Consider a
flask of E. coli cells growing on a medium containing the sugars glucose and
lactose . The cells exhaust the glucose and stop growing. For a short time it
appears that they cannot adjust to the new nutrient source; but then, after a
lag period of about an hour,growth resumes. During the lag, the cells have
been turning on the lac operon and beginning to accumulate the enzymes they
need to metabolize lactose. The growth curve is called “diauxic” from the
Latinauxilium, meaning help, because the two sugars help the bacteria grow.
First, the bacteria need an enzyme to transport the lactose into the cells. The
name of this enzyme is galactoside permease. Next, the cells need an enzyme
to break the lactose down into its two component sugars: galactose and
glucose.Because lactose is composed of two simple sugars,we call it a
disaccharide. These six-carbon sugars, galactose and glucose, are joined
together by a linkage called a b-galactosidic bond. Lactose is therefore called a
b-galactoside, and the enzyme that cuts it in half is called b-galactosidase.
The genes for these two enzymes, galactoside permease and b-galactosidase,
are found side by side in the lac operon, along with another structural gene—
for galactoside transacetylase—whose function in lactose metabolism is still
unclear.
, Diauxic growth: E. coli cells are grown on a medium containing both glucose
and lactose, and the bacterial density (number of cells/mL) is plotted versus
time in hours. The cells grow rapidly on glucose until that sugar is exhausted,
then growth levels off while the cells induce the enzymes needed to
metabolize lactose. As those enzymes appear, growth resumes.
The three genes coding for enzymes that carry out lactose metabolism are
grouped together in the following order: b-galactosidase (lacZ), galactoside
permease (lacY), galactoside transacetylase (lacA). They are all transcribed
together to produce one messenger RNA, called a polycistronic message,
starting from a single promoter.Thus, they can all be controlled together
simply by controlling that promoter. The term polycistronic comes from
cistron, which is a synonym for gene. Therefore, a polycistronic message is
One strategy bacteria employ to control the expression of their genes: by
grouping functionally related genes together so they can be regulated together
easily. Such a group of contiguous, coordinately controlled genes is called an
operon.
The first operon to be discovered has become the prime example of the
operon concept. It contains three genes that code for the proteins that allow
E. coli cells to use the sugar lactose, hence the name lac operon. Consider a
flask of E. coli cells growing on a medium containing the sugars glucose and
lactose . The cells exhaust the glucose and stop growing. For a short time it
appears that they cannot adjust to the new nutrient source; but then, after a
lag period of about an hour,growth resumes. During the lag, the cells have
been turning on the lac operon and beginning to accumulate the enzymes they
need to metabolize lactose. The growth curve is called “diauxic” from the
Latinauxilium, meaning help, because the two sugars help the bacteria grow.
First, the bacteria need an enzyme to transport the lactose into the cells. The
name of this enzyme is galactoside permease. Next, the cells need an enzyme
to break the lactose down into its two component sugars: galactose and
glucose.Because lactose is composed of two simple sugars,we call it a
disaccharide. These six-carbon sugars, galactose and glucose, are joined
together by a linkage called a b-galactosidic bond. Lactose is therefore called a
b-galactoside, and the enzyme that cuts it in half is called b-galactosidase.
The genes for these two enzymes, galactoside permease and b-galactosidase,
are found side by side in the lac operon, along with another structural gene—
for galactoside transacetylase—whose function in lactose metabolism is still
unclear.
, Diauxic growth: E. coli cells are grown on a medium containing both glucose
and lactose, and the bacterial density (number of cells/mL) is plotted versus
time in hours. The cells grow rapidly on glucose until that sugar is exhausted,
then growth levels off while the cells induce the enzymes needed to
metabolize lactose. As those enzymes appear, growth resumes.
The three genes coding for enzymes that carry out lactose metabolism are
grouped together in the following order: b-galactosidase (lacZ), galactoside
permease (lacY), galactoside transacetylase (lacA). They are all transcribed
together to produce one messenger RNA, called a polycistronic message,
starting from a single promoter.Thus, they can all be controlled together
simply by controlling that promoter. The term polycistronic comes from
cistron, which is a synonym for gene. Therefore, a polycistronic message is
