Control of enzyme amount: Transcriptional regulation of elongation/termination

Question 1

regulation of elongation and termination is often found in conjunction with

Answer 1

systems regulating transcription (1/2 component regulators)

normally back ups to these methods

Question 2

eg. the trp operon

Answer 2

encodes genes for making tryptophan

cell only wants to express when short of tryptophan. When tryptophan is not present, the TrpR cannot bind to the operator and prevent transcription

(the leader sequence in the operon occurs after the promotor and the operator and is a secondary checkpoint that transcription should be occurring)

Question 3

as wella s the TrpR repressor, the trp operon also uses a

Answer 3

row independent terminator

this is a type of transcriptional termination that includes a piece of mRNA with a GC rich stem loop and a run of U’s that cause transcription to terminate

Question 4

details of the row independent termination in trp operon

Answer 4

the mRNA for tryptophan encodes several potential stem-loop structures that can form regions: 1, 2, 3 and 4

as the mRNA is made, the first region to be transcribed is region 1. 1 can pair with 2, 2 can pair with 1 or 3, 3 can pair with 2 or 4.

because 1 is made first, 1 will always pair with 2 (and 3 with 4, which is the terminator) UNLESS 1 does not pair with 2 and 2 pairs with 3 instead. This happens when:

there is a shortage of trp: remember, in bacteria transcription and translation happen at the same time. the leader peptide has 2 trp codons, if there is not enough trp in the cell the ribosome will stall on these codons, which happen to be in region 1. this causes region 2 to pair with 3 instead, allowing transcription to continue :-)

so by default even if initiation and transcription occurs, it will terminate early and not code for protein

this is also the system in the isoleucine operon

possibly activate the stringent response?