Biology REGULATION OF GENE EXPRESSION

### KEY TOPICS

star Levels of Gene Regulation
star Conditions that regulate Expression
star Regulation in Prokaryotes

### LEVELS OF GENE REGULATION

● 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".

### CONDITIONS THAT REGULATE EXPRESSION

● 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.

### REGULATION IN PROKARYOTES

● 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.