Principles of Gene Expression & Regulation

Background
•Cells respond to different factors (environment, heat, food supply, and various external messages)same copy of genome in every cell in an organism, but genes are expressed at different levels in different cells
•Cells differentiate during development
•Gene expression determines what proteins are required at a particular point in development or location
•Gene regulation responsible for dynamic cells occur in different times and places (opening of chromatin, transcription, translation, protein stability, and protein modifications).strongest regulation occurs during transcription

Two types of prokaryotic regulation of transcription

•Negative control: active DNA binding protein represses transcription of the gene(s) downstream
Example: the Trp operon
•When Trp is present in the diet:
–Trp binds to the repressor protein
–This enables it to bind to the promoter
–This binding switches off the genes coding for the enzymes that make Trp
•This makes sense biochemically: it is energetically wasteful to synthesize an AA that is unneeded for bacterial growth
DNA binding proteins can also activate gene transcription

•Positive control: the active form of the DNA binding protein activates transcription downstream
–It is a transcriptional activator
•These proteins help to initiate transcription
–Help RNA pol to bind to the promoter
–Help to initiate transcription of bound, “stalled” RNA pol
Example: the lac operon
•Under both positive and negative control
•Its transcription only occurs if two conditions are met:
–Lactose present
–Glucose absent
•Note that these methods of transcriptional control involve DNA binding protein