Chapter 11: Regulation of Gene Expression (2nd half)
trpL mRNA has four regions that contain some complementarity with each other. what are the three possible stem-loop structures?
(1) If no translation, then 1-2 and 3-4 form and transcription terminates. (turns off) (2) If Trp levels are low, then 2-3 (but not 3-4) forms, leading to transcription. (turning of bc you need more trp) (3) If Trp levels are high, then 3-4 forms and transcription terminates. (turns off bc you have enough trp) trpL is transcribed first.
what are the two functions of trpL that are important in attenuation?
(1) Two Trp codons that act as a sensor to determine if the cell has sufficient Trp to synthesize its proteins. (2) The ability to form a stem-loop structure. two codons that are codons for trp are part of the leader seq. they decide if there is enough trp.
attenuation
(what makes trp different then lac is the process of attenuation) why trp is high trp is off bc of trpL which is a leader sep and attenuator seq.
what are the two things that can cause transcription to stop in trp
1. no translation 2. high trp
what happens when there is low trptophan levels
2-3 stem-loop forms Coupled transcription and translation occur under conditions in which the tryptophan concentration is very low. The ribosome pauses at the Trp codons in the trpL gene because insufficient amounts of charged tRNATrp are present. This pause blocks region 1 of the mRNA, so region 2 can hydrogen bond only with region 3. When this happens, the 3−4 stem-loop cannot form. Transcriptional termination does not occur, and RNA polymerase transcribes the rest of the operon. leader peptide is encoded by trpL
what happens when high trp levels?
3-4 stem-loop forms Coupled transcription and translation occur under conditions in which a sufficient amount of tryptophan is present in the cell. Translation of the trpL gene progresses to its stop codon, where the ribosome pauses. This blocks region 2 from hydrogen bonding with any region and thereby enables region 3 to hydrogen bond with region 4. This terminates transcription at the U-rich attenuator.
when tryptophan is present, trp operon is? via what two ways?
OFF 1. trytophan acts as a *corepressor* and binds to the repressor protein which then binds to the operator 2. attenuation when you have enough trp, trp operon is turned off by repressor. Trp acts as a coreporssor and binds to the repressor protein, which activiates it. That complex then binds to the operator so the polymeres cannot bind to the promoter
When tryptophan is absent, trp operon is?
ON via inactive reperssor what keeps it on it an inactive repressor
what is the leader seq?
The trp leader sequence (trpL) is a gene that encodes a peptide containing 14 amino acids called the leader peptide.
what happens when there is no translation?
When translation is not coupled with transcription, the most stable form of the mRNA occurs when region 1 hydrogen bonds to region 2 and region 3 hydrogen bonds to region 4. A terminator stem-loop forms, and transcription will be terminated just past the trpL gene. u-rich attenuator makes RNA poly. pop off therefore no translation -ex of rho independent and the enzymes that are part of trp are not transcribed.
what does trpR do?
makes repressor when trp is low cells need more trp so operator is on.
is the trp operon repressable or inducable?
repressable always off until turned on encodes 5 enzymes
