In the case when only lactose is available, the lac operon is turned on and maximally expressed. A small amount of lactose is converted to allolactose, which acts as an inducer by binding to the repressor protein and changing its conformation. This prevents the repressor protein from binding to the O site. In the absence of glucose, adenylyl cyclase is active and produces cAMP, which binds to the CAP. The cAMP-CAP complex then binds to the CAP site, leading to efficient transcription initiation by RNA polymerase at the promoter site. This positive regulation results in the production of a single polycistronic mRNA molecule containing three sets of start and stop codons. Translation of this mRNA produces the three proteins required for the cell to use lactose for energy production.

In the case when both glucose and lactose are available, the lac operon is uninduced and transcription is minimal, even if lactose is present in high concentrations. The presence of glucose inhibits adenylyl cyclase, preventing the formation of the cAMP-CAP complex and leaving the CAP site empty. As a result, RNA polymerase is unable to effectively initiate transcription, despite the repressor not being bound to the O site. This leads to low basal expression of the three structural genes of the operon.

The tryptophan (trp) operon contains five structural genes involved in the synthesis of the amino acid tryptophan. Similar to the lac operon, the trp operon is negatively controlled. In the case of the repressible trp operon, the negative control involves tryptophan itself binding to a repressor protein, facilitating the binding of the repressor to the operator. Tryptophan acts as a corepressor in this process. However, repression by tryptophan is not always complete, and the trp operon is also regulated through a process called attenuation. Attenuation allows transcription to be initiated but terminated before completion. If tryptophan is abundant, transcription initiation that escaped repression is stopped by the formation of an attenuator, which is a hairpin structure in the mRNA. This attenuation is similar to rho-independent termination. If tryptophan becomes scarce, the operon is expressed. The mRNA's 5'-end contains two adjacent codons for tryptophan. In the absence of tryptophan, ribosomes stall at these codons, covering regions of the mRNA required for the formation of the attenuation hairpin. This prevents attenuation and allows transcription to continue.

Transcriptional attenuation can occur in prokaryotes because translation of an mRNA begins before its synthesis is complete. This mechanism does not occur in eukaryotes due to the spatial and temporal separation of transcription and translation caused by the presence of a membrane-bound nucleus.

Coordination between transcription and translation is primary in bacteria, but regulation of ribosomal RNA (rRNA) and protein synthesis also plays crucial roles in adapting to environmental stress.
2.
When only lactose is available: In this case, the lac operon is induced (maximally expressed, or turned on). A small amount of lactose is converted to an isomer, allolactose. This compound is an inducer that binds to the repressor protein, changing its conformation so that it can no longer bind to the O site. In the absence of glucose, adenylyl cyclase is active, and cAMP is made and binds to the CAP. The cAMP–CAP transacting complex binds to the CAP site, causing RNA pol to initiate transcription with high efficiency at the promoter site (see Fig. 33.4B). This is an example of positive regulation. The transcript is a single polycistronic mRNA molecule that contains three sets of start and stop codons. Translation of the mRNA produces the three proteins that allow lactose to be used for energy production by the cell. [Note: In contrast to the inducible lacZ, lacY, and lacA genes, whose expression is regulated, the lacI gene is constitutive. Its gene product, the repressor protein, is always made and is active unless the inducer is present.] 3.
When both glucose and lactose are available: In this case, the lac operon is uninduced, and transcription is negligible, even if lactose is present at a high concentration. Adenylyl cyclase is inhibited in the presence of glucose (a process known as catabolite repression) so no cAMP–CAP complex forms, and the CAP site remains empty. Therefore, the RNA pol is unable to effectively initiate transcription, even though the repressor is not bound to the O site. Consequently, the three structural genes of the operon are expressed only at a very low (basal) level (see Fig. 33.4C). [Note: Induction causes a 50-fold enhancement over basal expression.]
D. Tryptophan operon
The tryptophan (trp) operon contains five structural genes that code for enzymes required for the synthesis of the amino acid tryptophan. As with the lac operon, the trp operon is subject to negative control. However, for the repressible trp operon, negative control includes Trp itself binding to a repressor protein and facilitating the binding of the repressor to the operator: Trp is a corepressor. Because repression by Trp is not always complete, the trp operon, unlike the lac operon, is also regulated by a process known as attenuation. With attenuation, transcription is initiated but is terminated well before completion (Fig. 33.6). If Trp is plentiful, transcription initiation that escaped repression by Trp is attenuated (stopped) by the formation of an attenuator, a hairpin (stem-loop) structure in the mRNA similar to that seen in rho-independent termination (see p. 437). [Note: Because transcription and translation are temporally linked in prokaryotes (see p. 454), attenuation also results in the formation of a truncated, nonfunctional peptide product that is rapidly degraded.] If Trp becomes scarce, the operon is expressed. The 5′-end of the mRNA contains two adjacent codons for Trp. The lack of Trp causes ribosomes to stall at these codons, covering regions of the mRNA required for formation of the attenuation hairpin. This prevents attenuation and allows transcription to continue.
Transcriptional attenuation can occur in prokaryotes because translation of an mRNA begins before its synthesis is complete. This does not occur in eukaryotes because the presence of a membrane-bound nucleus spatially and temporally separates transcription and translation.
E. Coordination of transcription and translation
Although transcriptional regulation of mRNA production is primary in bacteria, regulation of ribosomal RNA (rRNA) and protein synthesis plays important roles in adaptation to environmental stress.
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