lec 7.4 - transcription enhancers, elongation, and termination

enhancers

• can activate a promoter from upstream or downstream of promoter region of gene (from very far away)

• its sequence can be inverted relative to the promoter

• only work in cis

  • regulate gene expression of specific gene on same chromosome where they are located, cannot influence genes on other chromosomes

other initiation sequences/complexes involved in RNA pol II initiation - enhancer sequences and mediator complexes

• enhancer sequences - increase freq. of transcription from promoter

• mediator complexes - act as communication bridge btwn RNA pol II and other TFs

  • 20 proteins and 3 subdomains

bending DNA in RNA pol II initiation

• important factor for expression - transcription machinery can better access promoter region

• brings all components together

gene expression is associated with

• demethylation of promoter and regulatory regions

• determines which genes are on/off - gives an extra regulation mechanism

elongation

• continuous process until termination

• RNA pol holoenzyme reads the template strand from 3’ to 5’ and makes 5’ to 3’

how does termination vary in eukaryotes

• all 3 RNA pols in eukaryotes use different strategies for termination

pol I and pol III in termination

• these pols are similar to bacteria

  • pol I -terminates at termination sequence

  • pol III -terminates at T-rich sequence

why is a T-rich sequence good for termination?

• because it will bind to As and will only have 2 hydrogen bonds

• weaker

pol II in termination

• terminates several bases to several hundred bases away from 3’ end of mature mRNA

  • CPSF recognizes AAUAAA and cleaves RNA, then poly-A tail is added at 3’ end (needed to prevent degradation)

  • exonuclease removes rest of the RNA (gets rid of excess) and RNA pol is released from DNA

why is a poly-A tail added?

• to avoid degradation in cytosol

• more As in the tail = more stability

• fewer As = degraded quickly

• longer mRNA lasts = more protein made

poly A tail acts as an extra level of gene regulation/expression