生物化学原理教学（杨荣武）Chapter42 Prokaryotic regulation of gene expression.pptVIP

Chapter42 Prokaryotic regulation of gene expression Outline Negative vs positive regulation Regulation at the DNA level Regulation at the transcription level Regulation of Transcription Initiation: Operons( inducible/repressible) Regulation at Steps after Transcription Initiation-Attenuation and Antitermination Regulation at the translation level Regulation induced by environmental siganls Global regulation at the genome level Negative vs Positive Regulation Negative regulation—Protein (repressor) inhibits transcription (Ex. LexA). Inducer– binds to repressor, alters form, reduces affinity for target, allows expression of gene. Sometimes, small molecule required for repressor activity. Positive regulation—Activator protein increases transcription rate. Generally bound to a smaller signal molecule. THE PROBLEM Prokaryotes have to monitor its environment and accordingly change its metabolism Monitor pH, source of energy, galactose, amino acid residues, O2 Prokaryotes must accomplish specialized functions in one unspecialized cell Options Have all gene products functioning at all times (constitutive expression) Turn on genes only as they are needed (inducible expression) Gene expression is controlled by regulatory proteins In bacteria , signals are communicated to genes by regulatory proteins, which come in two types: activators and repressors. Many promoters are regulated by activators that help RNA polymerase bind DNA and by repressors that block that binding In the absence of both activator and repressor , RNA polymerase occasionally binds the promoter spontaneously and initiates a low level of transcription. The repressor blocks polymerase binding to the promoter. The site on DNA where a repressor binds is called an operator. An activator helps polymerase bind the promoter. this mechanism, often called recruitment, is an example of cooperative binding of proteins to DNA. Not all promoters are limited in the same way. In some cases, closed complex does not spo