L14 - prokaryotic gene expression 3

Question 1

why is transcriptional termination especially important in bacteria

Answer 1

bacterial DNA is nearly all coding

= continued transcription would code for another unwanted protein
= transcription must start and end effectively

Question 2

why is RNA polymerase decsribed as a ‘proscessive enzyme’

Answer 2

chance of continuing is higher than chance of protein falling off

Question 3

describe Rho-independant termination

Answer 3

intrinsic termination

formation of G-C rich hairpin loop in mRNA due to inverted repeat in DNA

followed by run of thymine bases that code for run of uracils in mRNA

= hairpin loop destabilises polymerase on the RNA
= run of uracil bases is ‘weak’ = polymerase disosociates

Question 4

what is Rho-independant termination dependant on

Answer 4

inverted repeat of G-C for hairpin

uracil/thymine sequnce close enough to hairpin

= hairpin without sequnce does not cause termination

Question 4

describe Rho-dependant termination of transcription in bacteria

Answer 4

Rho forms hexamer ring motor protein

binds to mRNA produced behind RNA polymerase and ‘chases’ it

pause sequnce causes polymerase to stop BUT Rho doesnt so catches it up

= pull RNA outof paused enzyme
= fuelled by ATP

Question 5

what is the controversy about Rho dependant termination

Answer 5

whether Rho is bound to RNA polymerase or not

= rest of the model is the same with Rho pulling mRNA out of polymerase

Question 6

what does Rho target to bind to

Answer 6

RUT sequnce

= Rho utilisation region

within mRNA produced not the DNA itself

Question 7

describe role of anti-termination in early and late genes

Answer 7

termination sequnces stop transcription after initiation

= when bound polymerase cannot get through

early genes in bacteria are synthesised but cannot bypass terminator sequnce

early genes code for antitermination protein
= binds to terminator

RNA polymerase can ‘read through’ transcription terminaor
= produce late genes

Question 8

what is Nut box in termination of transcription

Answer 8

N-utilsationn sequnce

N/anti-termination protein binds to Nut

= RNA polymerase transcribes past terminator sequnce

Question 9

how does N protein affect antitermination in terms of hairpin loop

Answer 9

prevents hairpin loop formation

= polymerase is not destabilises so does not fall off when it reaches run of thymines/uracils

Question 10

what are riboswitches

Answer 10

RNA sequnces that fold into stuctures regulated by binding of small metaboltes

= regulate transctiption by formation of intrinsic terminator upstream of gene
= hairpin loop