Gene Regulation ? Prokaryotes.docx

Gene Regulation ? Prokaryotes Prokaryotic gene regulation: immediate and/or nutritional needs Eukaryotic gene regulation: cell specialization Operon (prokaryotes) ? a set of genes under the control of a single operator and promoter, transcribed into a single mRNA Constitutive genes ? transcribed in a continually unregulated manner (independent of the presence of an inducer Inducible gene ? are turned on in the presence of an inducer (i.e. default state of gene is ?off?) Lac Operon ? transcription-level regulation Jacob and Monod (1961) Regulatory genes: code for repressor proteins Structural genes: code for enzymes and other proteins involved in metabolic pathways Repressor proteins ? bind tightly to DNA (coded for by the regulatory gene) Operator ?site on DNA where repressor proteins bind Promoter ? site on DNA where RNA polymerase wants to bind RNA polymerase binds to promoter sequence; DNA unwinds Lactose Absent/Lactose Present RNA polymerase can now bind to DNA and transcribe a series of enzymes that will break down lactose When lactose breaks down, it falls off the repressor protein ? which then re-gains its original shape and binds again to the DNA When tryptophan is present it binds to trp repressor, which alters its shape so that it binds to trp operator. No transcription occurs. When tryptophan is absent?repressor protein cannot bind to operator site. RNA polymerase can therefore bind and tr a scribe enzymes needed for tryptophan synthesis. A Ribosomal protein that, when made in excess, binds to mRNA and prevents further translation ? Translational control