The lac operon, an inducible operon, is a mechanism used by bacterial cells as an economical means to restrict the expression of the structural genes necessary for metabolizing lactose, a disaccharide. These structural genes break down lactose when lactose is the best carbon source available within its environs. The regulator protein regulates the operon. It turns the operon on or off depending upon the presence of lactose by binding to an operator region. The structural gene, Z, codes for-galactosidase which is primarily responsible for the hydrolysis of lactose into its monomeric units, galactose and glucose. 1 Operator – a segment of DNA that a repressor binds to. 2  It is classically defined in the lac operon as a segment between the promoter and the genes of the operon. 3  In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes. An operon contains one or more structural genes which are generally transcribed into one polycistronic mRNA (a single mRNA molecule that codes for more than one protein). However, the definition of an operon does not require the mRNA to be polycistronic, though in practice, it usually is. When glucose is present, the CRP does not bind to the promoter and expression of the lac operon remains low even if lactose is present. However, when glucose is absent, the CAP binds to the promoter region and promotes transcription. Glucose does not directly regulate the binding of CRP. A cAMP-CRP complex is entailed. The regulator protein regulates the operon. It turns the operon on or off depending upon the presence of lactose by binding to an operator region. The structural gene, Z, codes for-galactosidase which is primarily responsible for the hydrolysis of lactose into its monomeric units, galactose and glucose. To understand cis gene and before going ahead let us first learn what is operon (best example of cis-gene) ? An operon is a functioning unit of genomic DNA. This contains a cluster of genes which have a single point control.That single point control is called as regulatory signal or promoter. Lac Operon is the best example of Cis-gene. These genes are adjacent to each other (hint: means on same part of its own DNA-Cis gene) and has single point control (so called as operon) These are called as lacZ, lac Y and lac A respectively & together constitutes lac operon (Cis-gene). In constitutive mutants, the lac operon genes are continuously transcribed. A mutant regulator gene (lacI -) produces a defective repressor that cannot bind to the operator. A mutant operator region (lacO c) is defective in that the repressor cannot bind and thus transcription cannot be shut off. Both types of mutants have the same constitutive phenotype. In the absence of lactose the lac repressor blocks the expression of the lac operon by binding to the DNA at a site, called the operator that is downstream of the promoter and upstream of the transcriptional initiation site. The protein that inhibits transcription of the lac operon is a tetramer with four identical subunits called lac repressor. The lac repressor is encoded by the lacI gene, located upstream of the lac operon and has its own promoter. Expression of the lacI gene is not regulated and very low levels of the lac repressor are continuously synthesized. Genes whose expression is not regulated are called constitutive genes An operon is made up of 3 basic DNA components: 1  Promoter – a nucleotide sequence that enables a gene to be transcribed. 2  Operator – a segment of DNA that a repressor binds to. 3  Structural genes – the genes that are co-regulated by the operon. An operon contains one or more structural genes which are generally transcribed into one polycistronic mRNA (a single mRNA molecule that codes for more than one protein). However, the definition of an operon does not require the mRNA to be polycistronic, though in practice, it usually is. Therefore, Cis-gene is gene (part of DNA or RNA) which regulate (means control) the expression (means function) of genes which are on same DNA (on which cis gene itself is located) and cannot control genes expresion of genes which are on another DNA (which is another concept known as trans gene regulation). These genes are adjacent to each other (hint: means on same part of its own DNA-Cis gene) and has single point control (so called as operon) These are called as lacZ, lac Y and lac A respectively & together constitutes lac operon (Cis-gene). In the DNA of E. coli, there is a fourth gene, the regulator gene, lacI. The regulator gene is under separate transcriptional control from the other three genes. lacI codes for a protein called a repressor. The repressor binds to a region just before the lacZ gene. A mutant regulator gene (lacI -) produces a defective repressor that cannot bind to the operator. A mutant operator region (lacO c) is defective in that the repressor cannot bind and thus transcription cannot be shut off. Both types of mutants have the same constitutive phenotype.
http://userpages.umbc.edu/~lrowan1/lactoseoperon.html https://en.wikipedia.org/wiki/Operon http://userpages.umbc.edu/~lrowan1/lactoseoperon.html http://www.biotechnologyforums.com/thread-1544.html http://academic.emporia.edu/mooredwi/genetics/note16_1.htm http://chemistry.gravitywaves.com/CHEMXL153/RegulationofGeneExpression.htm https://en.wikipedia.org/wiki/Operon http://www.biotechnologyforums.com/thread-1544.html http://academic.emporia.edu/mooredwi/genetics/note16_1.htm
are operon genes cis acting