● `color{Violet}"Regulation"` of `color{Violet}"gene expression"` refers to a very broad term that may occur at `color{Violet}"various levels"`.

● Considering that gene expression results in the `color{Violet}"formation of a polypeptide"`, it can be regulated at several levels.

● In `color{Violet}"eukaryotes"`, the regulation could be exerted at:

(i) `color{Violet}"transcriptional level"` (formation of primary transcript)

(ii) `color{Violet}"processing level"` (regulation of splicing)

(iii) `color{Violet}"transport of mRNA"` from nucleus to the cytoplasm

(iv) `color{Violet}"translational level"`.

● The genes in a cell are expressed to perform a `color{Violet}"particular function"` or a `color{Violet}"set of functions"`.

● For example, if an enzyme called `color{Violet}"-galactosidase"` is synthesised by `color{Violet}"E. coli"`, it is used to catalyse the hydrolysis of a `color{Violet}"disaccharide"`, lactose into galactose and glucose; the bacteria use them as a `color{Violet}"source of energy"`.

● Hence, if the bacteria `color{Violet}"do not have lactose"` around them to be utilised for energy source, they would `color{Violet}"no longer require"` the synthesis of the enzyme `color{Violet}"beta-galactosidase"`.

● Therefore, in simple terms, it is the `color{Violet}"metabolic, physiological or environmental"` conditions that regulate the expression of genes.

● The development and differentiation of `color{Violet}"embryo into adult organisms"` are also a result of the `color{Violet}"coordinated regulation of expression"` of several sets of genes.

● In `color{Violet}"prokaryotes"`, control of the `color{Violet}"rate of transcriptional initiation"` is the predominant site for `color{Violet}"control of gene expression"`.

● In a `color{Violet}"transcription unit"`, the activity of `color{Violet}"RNA polymerase"` at a given promoter is in turn regulated by interaction with `color{Violet}"accessory proteins"`, which affect its ability to `color{Violet}"recognize start sites"`.

● These regulatory proteins can act both `color{Violet}"positively"` (`color{Violet}"activators"`) and `color{Violet}"negatively"` (`color{Violet}"repressors"`).

● The `color{Violet}"accessibility of promoter regions"` of prokaryotic DNA is in many cases regulated by the interaction of proteins with sequences termed `color{Violet}"operators"`.

● The operator region is `color{Violet}"adjacent"` to the promoter elements in `color{Violet}"most operons"` and in most cases the sequences of the operator bind a `color{Violet}"repressor protein"`.

● Each operon has its specific `color{Violet}"operator"` and specific `color{Violet}"repressor"`.

● For example, `color{Violet}"lac operator"` is present only in the `color{Violet}"lac operon"` and it interacts specifically with `color{Violet}"lac repressor"` only.