17: control of transcription

bacterial transcription

\-produce metabolic enzymes that are appropriate for the current environment

\-produce specific proteins during stress

bacterial transcription regulation

\-alternative sigma factors

\-enhancers that bind transcription factors

\-repressor proteins binding to DNA

\-transcription attenuation

\-control of multiple genes by one control site

temporal control

\-the activation of genes w in specific tissues at specific times during development

\-tissue specific gene expression

alternative sigma factors

\-phages (viruses that infect bacteria) and bacteria can exert control over which genes are expressed by producing different sigma subunits that direct rna polymerase to different genes

enhancers

\-dna sequences that enhance transcription

\-when bound to proteins, they are called transcription factors

\-ex. Fis sites, which bind upstream of genes to activate transcription

operon

\-a group of structural genes that are co transcribed, along with their operator, promoter, and any other control sites

operator

\-the main control site for an operon, usually a dna binding site for transcriptional repressor protein

\-promoter and operator sequences are physically adjacent to the structural genes

\-regulatory genes can be distant from the operon

lacZ

\-encoding beta galactosidase

\-e coli only makes beta galactosidase only if lactose is present

\-e coli prefers to use glucose as a carbon source, and will not make beta glactosidase if glucose is available

lacY

\-encodes lactose permease

lacA

\-encodes transacetylase

lacI

\-encodes the lac repressor protein

lac repressor

\-binds to lac operon (lacO) dna causing looping; no transcription

\-when bound to lactose, releases lacO dna and transcription begins

cap

\-adenylate cyclase produces cyclic AMP from ATP

\-cap binds to cAMP

\-cap-cAMP binds to cap site and stimulates transcription of lac operon

\-positive regulation

catabolite repression

\-lacO repressed by glucose

\-glucose inhibits adenylate cyclase, so cAMP is low

\-cap needs cAMP to stim the lacO transcription, so transcription is repressed

trp operon

\-encodes a leader sequence (trpL) and 5 enzymes that catalyze the multistep conversion of chorismate to tryptophan

trpR

\-controls the trp operon

\-binds to trpO when cellular levels of tryptophan are high, does not bind when levels are low

\-tryptophan is co repressor

\-attenuation related to leader sequence

pause structure

\-leader sequence structure pairing between regions 1 and 2

terminator structure

\-leader sequence structure pairing regions 3 and 4 (rho independent)

\-formed when tryptophan levels are high, and transcription stops

antiterminator structure

\-leader sequence structure w alternative pairing between regions 2 and 3

\-forms when tryptophan levels are low and the ribosome stalls at the 2 trp codons, and transcription proceeds

trap

\-a protein involved in trp attenuation

\-in the presence of trp, the protein binds to rna

\-binding prevents formation of antiterminator

\-when protein is bound, trp operon is not transcribed