Question

15. Describe the gene expression at the lac operon in the following conditions. Please discuss all components involved and how they regulate the expression:

a. Absence of glucose; presence of lactose

b. Absence of lactose; presence of glucose

Answer 1

operon is a unit of bacterial gene expression and regulation, which includes structural genes and regulatory sequences recognized by regulatory gene products. all the genes in an operon are expressed as a single unit. single promoter is required to initiate and regulate transcription of all structural genes.

the product of lac operon involved in conversion of disaccaride sugar lactose into its monosaccharide glucose and galactose.

lac operator - short region of DNA that lies between promoter and structural genes and interacts with regulatory proteins which controls the transcription of operon.

primary operator (O1) and - palindromic sequence of 26bp from -5 to +21. cis acting site is present on DNA to which lac repressor binds. when the lac repressor binds at the operator it prevents RNA polymerase from initiating transcription at the promoter.

secondary operators(O2 and O3)- located on either side of primary operators. lesser affinity for repressor.

lac structural genes - any gene that codes for protein or RNA. lactose is beta- galatoside that e.coli use for energy and as a carbon source.

lac Z - codes for enzyme beta-galactosidase. cleaves lactose into glucose and galactose.

lac Y- codes for beta galactoside permease. transpors lactose into cell.

lac A codes for beta -galactoside transacetylase. play role in detoxification of compounds by transfering acetyl group from acetyl coA to beta-galactosides.all the threee genes are transcribed into single multigenic mRNA molecule.

lac repressor -

product of trans acting regulatory gene. regulating gene- gene that codes for a protein or RNA involved in regulating the expression of other genes.

DNA binding site of repressor binds with lac operator.I gene encodes lac repressor. block expression of structural genes. it is homotetramer, 4 identical subunits , molecular weight- 38000. two recognition sites - DNA binding site and allosteric site that binds to actose analogs.

when the repressor binds the lactose or analogs, the protein undergo conformational changes, change in DNA binding site, so that the repressor no longer has high affinity for the operator . thus, in response to binding lactose, the repressor falls off the DNA.

POSITIVE REGULATION ----

when e.coli finds both glucose and lactose in the medium, it metabolizes the glucose and represses the use of lactose. result is diauxic growth. the term catabolite repression is used for this type of glucose effect.

it caused by reduction in level of intracellular cAMP. CRP(catabolite gene activator protein) act as positive regulator protein and activated by binding of cAMP.cAMP is synthesised by adenylate cyclase. when glucose is high cAMP is low.

cAMP-CRP complex must be bound to a specific base sequence called activation site in order for trancription to ocur.