#Question id: 2860

 Lactose metabolism in E. coli is carried out by two enzymes, with possible involvement by a third. The genes for all three enzymes are clustered together and transcribed together from one promoter, yielding a polycistronic message. These three genes, linked in function, are therefore also linked in expression. They are turned off and on together.

Negative control	keeps the lac operon repressed in the absence of lactose.
positive control	keeps the operon relatively inactive in the presence of glucose, even when lactose is present.
	when all the glucose is gone and lactose is present, a mechanism should exist.
	is mediated by a factor called catabolite activator protein (CAP), which, in conjunction with cyclic-AMP, stimulates transcription.