Gene expression in prokaryotes is most commonly regulated through control of transcription. An operon consists of a set of closely linked structural genes and a set of DNA sequences (promoter and operator) that control their transcription. Operons can be regulated by both negative and positive controls. Sigma factors control the expression of specific classes of prokaryotic genes that share recognition sequences in their promoters.
learning outcomes
You should be able to:
Analyze conditions that affect expression of genes in the lac operon.
Describe the roles of various DNA sequences in the lac operon.
Explain how sigma factors and genetic sequences are involved in gene regulation in prokaryotes.
A prokaryotic cell can metabolize sugar “X,” using the enzyme “Xase.” When there is a low concentration of X in the environment, there is low activity of Xase in the cells; but when the X concentration is high, Xase is also high. What five mechanisms could the cell use to reduce the activity of Xase in the absence of X?
The cell could reduce the transcription of the Xase gene, hydrolyze Xase mRNA, prevent Xase RNA translation, hydrolyze Xase protein, or inhibit Xase protein.
Compare the molecular conditions at the lac operon promoter in the presence versus absence of lactose.
In the presence of lactose, the promoter is exposed and RNA polymerase binds to begin transcription. In the absence of lactose, the promoter is occupied by the repressor and there is no transcription.
How do sigma factors and recognition sequences act to affect the expression of classes of genes?
Sigma factors direct RNA polymerase to bind to recognition sequences at certain promoters. If several promoters of genes with related functions (e.g., heat shock response) have the same recognition sequence, transcription of those genes will occur at the same time and rate.
The repressor protein that acts on the lac operon of E. coli is encoded by a regulatory gene. The repressor is made in small quantities and at a constant rate. Would you surmise that the promoter for this repressor protein is efficient or inefficient? Is synthesis of the repressor constitutive, or is it inducible and under environmental control?
To keep a constant, low-
Studies of bacteria have provided a basic understanding of mechanisms that regulate gene expression and of the roles of regulatory proteins in both positive and negative regulation. You’ll see these same types of mechanisms again as we now turn to the transcriptional control of gene expression in eukaryotes.