{"id":596,"date":"2023-04-01T16:12:01","date_gmt":"2023-04-01T16:12:01","guid":{"rendered":"https:\/\/pressbooks.justwrite.in\/understanding-gene-regulation\/?post_type=chapter&p=596"},"modified":"2023-04-09T09:22:08","modified_gmt":"2023-04-09T09:22:08","slug":"prokaryotic-gene-regulation-lac-operon","status":"publish","type":"chapter","link":"https:\/\/pressbooks.justwrite.in\/understanding-gene-regulation\/chapter\/prokaryotic-gene-regulation-lac-operon\/","title":{"raw":"Prokaryotic gene regulation- Lac Operon","rendered":"Prokaryotic gene regulation- Lac Operon"},"content":{"raw":"

Lactose Operon: An Inducible Operon<\/span><\/h1>\r\nFrancois Jacob & Jacques Monod. studied the mechanisms of transcriptional regulation in E. coli.\u00a0 \u00a0<\/em><\/span>\r\n\r\nThe lac operon consists of\u00a0 two main components\u00a0 namely\u00a0 the regulatory genes<\/strong> and\u00a0 structural genes .<\/strong><\/em><\/span>\r\n\r\nThe structural genes<\/em><\/strong>\r\n\r\nThe structural genes are lax Z, lac Y and lac A. These encode the three enzymes involved in lactose metabolism ; These are\u00a0 lac Z e<\/i><\/b>ncodes an enzyme called<\/span>\u03b2-galactosidase<\/strong>, which digests lactose into its two constituent sugars namely\u00a0 glucose and galactose.\u00a0<\/span>\u00a0lac Y <\/strong>encodes the enzyme called \u00a0<\/em><\/span>permease <\/strong>that helps to transfer lactose into the cell and lac A<\/strong>\u00a0 encodes <\/span>\u00a0<\/em>trans-acetylase<\/strong>; the relevance of which in lactose metabolism is not clearly understood.<\/span>\r\n\r\nThe regulatory genes\u00a0<\/strong>\r\n\r\nIn addition to the three protein-coding genes, the\u00a0<\/span>lac<\/em> operon contains short DNA sequences that do not encode proteins,\u00a0\u00a0<\/span>These sequences are called <\/span>P (promoter)<\/strong>,\u00a0<\/span>O (operator)<\/strong>, and\u00a0<\/span>CBS (CAP-binding site) <\/strong>. These are\u00a0<\/span>\u00a0binding sites for proteins involved in transcriptional regulation of the operon. <\/span>\r\n\r\n\u00a0These sequences are called <\/span>cis<\/em>-elements<\/strong> as they are <\/span>located on the DNA of\u00a0 the genes they regulate. On the other hand, the proteins that bind to these <\/span>cis<\/em>-elements are called\u00a0<\/span>trans<\/em>-regulators<\/strong> . These are diffusible molecules and are not necessarily need to be encoded on the same piece of DNA as the genes they regulate. <\/span>\r\n\r\nOne of the major\u00a0trans<\/em>-regulators of the\u00a0lac<\/em>\u00a0operon is encoded by\u00a0lacI<\/strong>. which codes for the lac I protein. <\/em>Four identical molecules of\u00a0lacI<\/em>\u00a0proteins assemble together to form a\u00a0homotetramer<\/strong>\u00a0called a\u00a0repressor.<\/strong><\/span>\r\n\r\nNegative regulation of the Lac Operon\u00a0<\/strong>\r\n\r\nIn the absence of Lactose and when glucose is available in the medium , the repressor binds to two operator sequences adjacent to the promoter of the\u00a0<\/span>lac<\/em>\u00a0operon. Binding of the repressor prevents RNA polymerase from binding to the promoter <\/span>. Therefore, the operon will not be transcribed when the operator is occupied by a repressor. This is called negative regulation.<\/span>\r\n\r\n \r\n\r\nPositive Regulation\u00a0<\/strong>\r\n\r\nWhen\u00a0 the level of glucose in the medium is\u00a0 \u00a0very low or non-existent and when lactose is available in the medium, the lac operon is induced to express the genes i.e., Only when glucose is absent and lactose is present will the <\/span>lac<\/em> operon be transcribed .When lactose is present, its metabolite, allolactose<\/strong>, binds to the\u00a0lac<\/em>\u00a0repressor and changes its shape so that it cannot bind to the\u00a0lac<\/em> operator to prevent transcription.\u00a0 This is referred\u00a0to as positive regulation<\/span>\r\n\r\n\u00a0It should be mentioned that the lac operon is transcribed at a very low rate even when glucose is present and lactose absent.<\/span>\r\n\r\n[caption id=\"attachment_634\" align=\"aligncenter\" width=\"469\"]\"Picture Transcription regulation of the lac operon<\/a> from Biology<\/em>, an OpenStax resource <\/span><\/a>is licensed under\u00a0CC BY-SA 4.0<\/a><\/span><\/span><\/span><\/span>[\/caption]\r\n\r\n \r\n\r\nPositive Regulation by CAP\u00a0<\/strong>\r\n\r\n\u00a0CAP is a trans<\/em>-factor called\u00a0<\/span>cAMP binding protein.\u00a0<\/strong> CAP protein binds to a\u00a0 cis element within the lac promoter called CAP binding sequence (CBS). \u00a0<\/strong> CBS is located very close to the promoter (P).\u00a0 CAP can bind to CBS only after it is binds with cAMP .When CAP is bound to at CBS, RNA polymerase is better able to bind to the promoter and initiate transcription. Thus, the presence of cAMP ultimately leads to a further increase in lac <\/em>operon transcription.\r\n\r\nThe concentration of cAMP is inversely proportional to the abundance of glucose:\u00a0 when glucose concentrations are low, the enzyme called\u00a0<\/span>adenylate\u00a0 \u00a0 \u00a0 \u00a0cyclase<\/strong> \u00a0produces cAMP from ATP.E. coli<\/em>\u00a0prefers glucose over lactose, and so expresses the\u00a0lac<\/em>\u00a0operon at high levels only when glucose is absent and lactose is present. \u00a0The lac operon expressed at its highest levels only in the presence of lactose, and in the absence of glucose .<\/span>\r\n\r\n\"Picture\r\n

\"cAMP binds to the CAP protein, a positive regulator\"<\/a> from Biology<\/em>, an OpenStax resource <\/a>is licensed under\u00a0CC BY-SA 4.0<\/a><\/span><\/span><\/p>\r\n\r\n\r\n

\r\n\r\nHere is a simulation on Gene machine-The lac operon<\/a>. After playing with this simulation you will be able to\r\n