Mechanism of regulation:

A number of protein-coding genes are active in all cells and are needed for so- called house-keeping functions like as the enzymes of glycolysis, the citric acid cycle and the proteins of the electron transport chain. Moreover, some genes are active only in specific cell kinds and are responsible for defining the specific characteristics  and function of those cells; for instance  immunoglobulin  genes in lymphocytes, myosin in muscle cells. Additionally, the proteins expressed  through any given  cell may modify over time (for instance  during  early development)  or in response  to external stimuli, like as hormones. The Eukaryotic cells can regulate the expression of protein-coding genes at a number of levels but a prime site of regulation is transcription.

Transcriptional regulation in a eukaryotic cell that genes are transcribed and at what rate) is mediated  through transcription  factors (other than the common  transcription  factors) that recognize and bind to short regulatory  DNA sequences  (control  parts)  related with the gene. These sequences are also known as cis-acting elements (or simply cis-elements) since they are on the similar DNA molecule as the gene being controlled (cis is Latin for 'on this side'). The protein transcription  factors which bind to these parts are also known as trans-acting factors (or  simply  trans-factors) in which the genes encoding  them  can  be on variant DNA molecules instance for on different chromosomes. The transcription  factors  that  regulate  specific  gene transcription do so through interacting with the proteins of the transcription initiation complex and should either  increase  (activate) or decrease (repress) the rate of transcription of the  goal  gene. Classically each  protein-coding gene in a eukaryotic cell has various control elements in its promoter and therefore is under the control of various  transcription  factors  that interact with each other and with the transcription initiation complex through protein: the protein  interaction  to determine  the rate of transcription of that gene.

Figure:  Control regions that regulate transcription of a typical eukaryotic protein-coding gene. Although shown as distinct entities here for clarity, in vivo the different regulatory proteins bound to the control elements and distant enhancers interact with each other and with the general transcription factors of the transcription initiation complex to modulate the rate of transcriptional initiation.