16.3 genetics quiz

attenuator sequence

a sequence found in certain operons (trp operon) in bacteria that stops transcription soon after it has begun

the lac operon

an inducible operon regulated by sugar molecules that activate transcription (catabolic pathway)

attenuation

can also occur in high tryptophan levels

high tryptophan levels

corepressor- repressor binds to operator and blocks transcription. repressor becomes active

mRNA can form stem loops. the types of stem loops that occur

determine whether or not attenuation occurs

trpR gene

encodes the trp repressor protein

trpE, trpD, trpC, trpB, trpA

genes encode enzymes involved in tryptophan biosynthesis

trp operon

is a repressible operon regulated by tryptophan, a corepressor that binds to the repressor and turns the operon off. (anabolic pathway)

trpL

plays a regulatory role

when translation is not coupled with transcription

region 1 hydrogen bonds to region 2 and region 3 hydrogen bonds to region 4. Because a 3-4 terminator stem-loop forms, transcription will be terminated at the U-rich attenuator.

in trp operator

repressor is NOT automatically bound to the operon. must have a corepressor in order for it to inhibit transcription (in lac operon repressor is automatically bound to the operon and needs an activator to take it off of the operon and allow transcription)

3-4 stem loop

results in intrinsic termination (rho independent). terminates transcription at the u-rich attenuator.

coupled transcription and translation occur when 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 structure cannot form. Attenuation does not occur, and RNA polymerase transcribes the rest of the operon

low tryptophan levels

transcription of the entire trp operon occurs

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 levels within the cell are very low

trp repressor cannot bind to the operator site. transcription of the trp operon takes place and cell expresses the genes required for synthesis of tryptophan

when tryptophan levels within the cell become high

tryptophan acts as a corepressor that binds to the trp repressor that allows it to bind the the trp operator site. This binding inhibits the ability of RNA polymerase to transcribe the operon. Therefore, when a high level of tryptophan is present within the cell-when the cell does not need to make more tryptophan-the trp operon is turned off