Structural genes are the nucleotide sequences of
DNA that serve as templates for the synthesis of
RNAs. The average length of structural genes specifying
proteins in prokaryotes is about 1,000 bp, compared
to about 10,000 bp in eukaryotes. Structural
genes and the controlling sites that regulate the rate
of transcription of these genes are called operons
Regulatory proteins are proteins that affect the expression of structural
genes by binding to controlling sites near the structural genes and
either activating or repressing transcription. Regulatory proteins that
stimulate gene transcription are termed transcription factors or activators.
Repressors are proteins that inhibit the initiation of transcription
when bound to controlling sequences called operators. Proteins that terminate
transcription are referred to as terminator proteins. However, the
end of transcription is most often signaled by a specific terminator sequence
found in the DNA or newly transcribed RNA. In general, activators stimulate RNApolymerase binding to promoter sites on DNA at the
beginning of structural genes, whereas repressors inhibit RNA polymerase
binding. Controlling sites are short nucleotide sequences of
DNA , usually 15-30 bp long, that control the expression of structural
genes next to them.
The simplest operon consists of one structural gene and one promoter
to serve as a binding site for RNA polymerase. These operons are constitutive,
that is, they are expressed at all times. The regulatory operon in
Figure 4-1 is an example of a simple operon. Some simple operons may
be regulated by an attenuator, which codes for RNAstructure that causes
the RNA polymerase to prematurely cease transcribing.
|Figure 4-1 Structure of the lactose operon.
Most operons in bacteria consist of numerous structural genes and
controlling sites. In bacteria, many operons contain more than one structural
gene, or cistron. These polycistronic operons are transcribed into
a single mRNA. Each protein-coding region in the mRNA is defined by
its own start codon, where protein synthesis is initiated, and a nonsense
codon, where protein synthesis is terminated. Operons in eukaryotes are
usually monocistronic, that is, they contain a single gene. A regulon is a
group of operons under the control of a regulatory protein. The operons
in a regulon are generally not contiguous.