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''lac'' operon (lactose operon) is an operon required for the transport and metabolism of lactose in ''Escherichia coli'' and many other enteric bacteria. Although glucose is the preferred carbon source for most bacteria, the ''lac'' operon allows for the effective digestion of lactose when glucose is not available. Gene regulation of the ''lac'' operon was the first genetic regulatory mechanism to be understood clearly, so it has become a foremost example of prokaryotic gene regulation. It is often discussed in introductory molecular and cellular biology classes at universities for this reason. Bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA transcript. In this case, when lactose is required as a sugar source for the bacterium, the three genes of the lac operon can be expressed and their subsequent proteins translated: ''lacZ'', ''lacY'', and ''lacA''. The gene product of ''lacZ'' is β-galactosidase which cleaves lactose, a disaccharide, into glucose and galactose. LacY encodes lactose permease, a protein which becomes embedded in the cytoplasmic membrane to enable transport of lactose into the cell. Finally, ''lacA'' encodes galactoside O-acetyltransferase. It would be wasteful to produce the enzymes when there is no lactose available or if there is a more preferable energy source available, such as glucose. The ''lac'' operon uses a two-part control mechanism to ensure that the cell expends energy producing the enzymes encoded by the ''lac'' operon only when necessary. In the absence of lactose, the ''lac'' repressor halts production of the enzymes encoded by the ''lac'' operon. In the presence of glucose, the catabolite activator protein (CAP), required for production of the enzymes, remains inactive, and EIIAGlc shuts down lactose permease to prevent transport of lactose into the cell. This dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases, known as diauxie. == Structure of the ''lac'' operon == *The ''lac'' operon consists of three structural genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are: ''lacZ'', ''lacY'', and ''lacA''. * * ''lacZ'' encodes β-galactosidase (LacZ), an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose. * * ''lacY'' encodes lactose permease (LacY), a transmembrane symporter that pumps β-galactosides into the cell using a proton gradient in the same direction. * * ''lacA'' encodes galactoside O-acetyltransferase (LacA), an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides. Only ''lacZ'' and ''lacY'' appear to be necessary for lactose catabolism. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「lac operon」の詳細全文を読む スポンサード リンク
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