10 THE OPERON (Full Edition)
3 Structural gene clusters are coordinately controlled
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- An operon is a unit of bacterial
gene expression and regulation, including structural genes and control
elements in DNA recognized by regulator gene product(s).
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Genes coding for proteins that function in the same pathway may be located
adjacent to one another and controlled as a single unit that is
transcribed into a polycistronic mRNA.
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Bacterial structural genes are often
organized into clusters that include genes coding for proteins whose
functions are related. It is common for the genes coding for the
enzymes of a metabolic pathway to be organized into such a cluster. In
addition to the enzymes actually involved in the pathway, other related
activities may be included in the unit of coordinate control; for
example, the protein responsible for transporting the small molecule
substrate into the cell.
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Figure 10.4
The lac operon occupies ~6000 bp of DNA. At the left the lacI
gene has its own promoter and terminator. The end of the lacI region is
adjacent to the promoter, P. The operator, O, occupies the first
26 bp of the transcription unit. The long lacZ gene starts at base 39,
and is followed by the lacY and lacA genes and a terminator.
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The cluster of the three lac structural genes, lacZYA, is typical.
Figure 10.4 summarizes the organization of the structural genes, their associated cis-acting
regulatory elements, and the trans-acting regulatory gene. The
key feature is that the cluster is transcribed into a single
polycistronic mRNA from a promoter where initiation of transcription is
regulated.
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The protein products enable cells to take up and metabolize
ß-galactosides, such as lactose.
The roles of the three structural genes are:
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lacZ codes for the enzyme ß-galactosidase, whose active form is a tetramer of ~500 kD.
The enzyme breaks a ß-galactoside
into its component sugars. For example, lactose is cleaved into glucose
and galactose (which are then further metabolized).
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lacY codes for the ß-galactoside permease, a 30 kD membrane-bound protein constituent of the transport system. This transports ß-galactosides into the cell.
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lacA codes for ß-galactoside transacetylase, an enzyme that transfers an acetyl group from acetyl-CoA to ß-galactosides.
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Mutations in either lacZ or lacY can create the lac
genotype, in which cells cannot utilize lactose. (The genotypic description "lac"
without a qualifier indicates loss-of-function.) The lacZ mutations abolish enzyme activity,
directly preventing metabolism of lactose. The lacY mutants cannot take up lactose from the medium.
(No defect is identifiable in lacA
cells, which is puzzling. It is possible that the acetylation reaction
gives an advantage when the bacteria grow in the presence of certain
analogs of ß-galactosides that cannot be metabolized,
because the modification results in detoxification and excretion.)
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The entire system, including structural
genes and the elements that control their expression, forms a common
unit of regulation; this is called an operon. The activity of the operon
is controlled by regulator gene(s), whose protein products interact with the
cis-acting control elements.
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© Jones and Bartlett Publishers (2007)
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