Gene expression regulation in

prokaryotes

Constitutive versus regulated enzymes

inducible enzymes – catabolic

pathways – complex sugar

metabolism

repressible enzymes – anabolic

pathways (amino acid synthesis)

Regulated expression of enzymes result in

energy savings

Discoveries leading to the operon

model of regulation of gene expression

in bacteria

(lac operon)

• Genes that code for enzymes with related functions tend to be organized in clusters in the chromosome and transcribed as a single unit

• Induction of these protein coding (structural) genes depends on regulatory regions

• Constitutive mutations led to the discovery of two regulatory genes:

lacI – which maps at a distance to the structural genes

lacO – located immediately before the first of the structural genes

lac operon and its regulatory elements

Jacob and Monod’s model

Wild type E.coli lac operon – no lactose

Wild type E.coli lac operon – lactose present

Effect of lacOc mutation in partial diploid

Genetic evidence for operon model - merozygotes

Effects of lacI- mutation

Dominant effect of lacIs mutation

Table 16-1 Copyright © 2006 Pearson Prentice Hall, Inc.

Positive control – Role of cyclic AMP

Tryptophan operon – a repressible operon

Attenuation - alternative structures

trp operon leader mRNA

Attenuation - alternative

structures trp operon

leader mRNA

Models for attenuation

of the trp operon