week 3 - transcriptional regulation - prokaryotes Flashcards
why is gene expression regulated in prokaryotes
-gene expression is an elaborate and energy consuming multistep process
-prokaryotes only express genes ‘if and when’ to conserve energy
‘if and when’ relates to genes:
-that are required for ‘housekeeping’
-that are ‘switched on or off’ in response to changes in environmental conditions
how is gene expression regulated in prokaryotes
gene expression can be regulated at various points:
- regulate transcription: predominant in prokaryotes
-hydrolyze mRNA, preventing translation
-prevent mRNA transaction at the ribosome
-degrade the protein after it is made
-inhibit the function of the protein
transcriptional regulation steps
- gene expression begins at the promote
- not all genes are transcribed at any given time - in selective gene expression a ‘decision’ is made about which genes to activate
- two types of regulatory proteins may bind DNA : repressor proteins and activator proteins
what is the role of the sigma factor
sigma factor recognises and binds core promoter elements (-10 and -35 sequences) and recruits RNA polymerase
core promoter element variants are recognised and bound by specific sigma factors
‘strong’ promoters conform to the consensus sequence and initiate transcription frequently (RNA synthesis level determined by initiation and elongation of transcription)
what are two ways to regulate a metabolic pathway
- allosteric regulation of enzyme - catalysed reactions allow rapid fine-tuning
- regulation of gene expression is slower but conserves resources
a typical operon consists of
a promoter
2 or more structural genes
an operator - short stretch of DNA between the promoter and structural genes
how is transcription regulated in prokaryotes
an inducible operon regulated by a repressor protein (default is off)
a repressor operon regulated by a repressor protein (default is on)
an operon regulated by an activator protein
