Regulation by a Cascade in Phages
In phages, the early, middle (or delayed early) and late genes are expressed in an order and a study of genetic map suggests, that genes are arranged on the map in the order in which they are expressed. This, therefore, needs smallest numher of switches and the regulation is exercised with a maximum economy. In the very early genes, the transcription apparatus of the host is utilised and the product is then used for initiating the transcription of the next class of genes (delayed early or middle genes). The transcription of these middle genes will be necessary for expression of late genes through a regulator, such that these successive controls (where expression of each set is necessary for the expression of the next set of genes) creates a cascade, in which a group of genes are turned on or off at a particular time.