16.3 Gene Control

A structural gene encodes a functional product such as an enzyme or structural protein. A regulatory gene encodes a product or control sequence that changes the expression of other genes.

Separating product from control lets cells adjust pathway activity without changing every structural gene. Regulation can act at transcription, RNA processing, translation or protein activity.

A regulatory protein can bind near a structural gene and prevent RNA polymerase from transcribing an enzyme that is not currently needed.

‘Regulatory’ does not mean the gene has no product. It means its product or sequence influences expression of another gene or set of genes.

An inducible system is usually off until a substrate or signal removes repression and allows enzymes to be made. A repressible system is usually on until an end product activates repression and prevents unnecessary synthesis.

Both designs conserve energy by matching enzyme production to pathway need. The same word ‘repressor’ can participate in opposite logic depending on the signal and operator state.

A substrate may bind a repressor and stop it binding DNA in an inducible pathway; an accumulated product may bind a repressor and enable it to block a biosynthetic pathway.

Inducible does not mean permanently active, and repressible does not mean permanently off. State the starting state and the signal that changes it.

In a prokaryotic operon, a promoter, operator and linked structural genes are transcribed together. A regulatory protein can bind the operator and prevent RNA polymerase from transcribing the structural genes.

Promoter: RNA-polymerase binding site; operator: regulatory switch; structural genes: pathway products; regulator gene: produces the repressor or activator; a small molecule can alter regulator activity.

Co-transcription ensures enzymes in the same pathway are produced together and reduces wasted resources. The operator connects a signal to a whole group of genes.

When a required substrate is absent, a repressor can block the operator; when the substrate appears, it changes the repressor so transcription proceeds.

The operator is DNA, not an enzyme, and the structural genes do not each need a separate promoter in a simple operon model.

Transcription factors are proteins that bind specific regulatory DNA sequences and influence RNA-polymerase recruitment or activity. Activators increase transcription; repressors reduce it, often through interactions with other proteins and chromatin.

Multiple factors allow a eukaryotic cell to combine developmental, hormonal and environmental signals. The same gene can therefore be active in one tissue and silent in another.

A hormone-bound receptor can enter the nucleus, bind a response element and recruit co-activators, increasing transcription of a target gene.

A transcription factor does not necessarily bind the promoter itself. Enhancers, silencers, chromatin state and cofactors can all affect the final outcome.

Gibberellin binds a receptor and initiates signalling that removes repression of target genes. In barley, this can activate amylase production; in stems, it can activate proteins associated with elongation.

The hormone changes gene expression rather than acting as a structural component of the growing tissue. Different targets and tissues explain why one hormone can produce distinct responses.

Adding gibberellin to an aleurone layer can increase amylase mRNA and enzyme activity, releasing soluble sugars from endosperm starch.

Gibberellin does not directly convert DNA into enzyme. It changes regulatory protein activity, which changes transcription of target genes.