Five E. coli strains have been identified, each of which has a different mutation that disrupts the normal regulation of a particular operon. For each mutant strain, the mutation has been mapped to the promoter or the operator region; however, the exact sequence changes are not known for these mutations. It is known that the normal promoter/operator consists of a single binding site for a positively acting transcription factor located just upstream of the promoter itself. Short DNA fragments containing the promoter and the operator were subcloned from each of the five mutant strains and from the wild type, purified, and radiolabeled. These fragments were then incubated under conditions of DNA excess with either purified regulatory factor or RNA polymerase or with both polymerase and regulatory factor.

The resulting protein-DNA complexes were separated by electrophoresis, and the radioactive DNA fragments were detected by exposure to x-ray film, giving the results shown below. Electrophoresis is from top to bottom; the largest complexes run slowest.

Based on above experiment, match the following

Effect	Mutant
1. One of the mutations increases the affinity of the polymerase for the promoter. Transcription of the operon is not stimulated by the regulatory factor in this mutant.	A. Mutant 5
2. One of the mutations maps to the operator. Transcription of the operon is not stimulated by the regulatory factor in this mutant. Which mutant is most likely to show this effect	B. Mutant 2
3. One of the mutations is known to result from a small deletion between the operator and the promoter. The polymerase and the regulatory factor is each able to bind to the mutated DNA sequence, but are unable to form the three components complex. Transcription of the operon is not stimulated by the regulatory factor in this mutant. Which mutant shows the properties that might be expected for such a change?	C. Mutant 4
	D. Mutant 3
	E. Mutant 1

#Question id: 4296

#Unit 3. Fundamental Processes

 Normally, a stop codon is required to release the ribosome from an mRNA but what happens to a ribosome that initiates translation of an mRNA fragment that lacks a termination codon. The Ribosome stalled at broken part of mRNA. In prokaryotic cells, such stalled ribosomes are rescued by the action of a chimeric RNA molecule that is part tRNA and part mRNA, appropriately called a tmRNA. What is sequence of events of this rescue process.

A. Translocation of the peptidyl-SsrA RNA results in the release of the broken mRNA

B. the SsrAAla–EF-Tu–GTP complex binds to the A-site of the ribosome and participates in the peptidyl transferase reaction

C. portion of the SsrA RNA acts as an mRNA and encodes 10 codons followed by a stop codon

D. the protein encoded by the incomplete mRNA is fused to a 10-amino-acid “peptide tag” at its carboxyl terminus, and the ribosome is recycled. Interestingly, the 10-amino-acid tag is recognized by cellular proteases that rapidly degrade the tag and the truncated polypeptide to which it is attached

#Question id: 4297

#Unit 3. Fundamental Processes

Match the following decay mechanism

#Question id: 4298

#Unit 3. Fundamental Processes

Insulin and other growth factors stimulate a pathway involving a protein kinase mTOR, which in its turn augments protein synthesis. mTOR essentially modifies protein(s) which in their unmodified form act as inhibitors of protein synthesis. The following protein is NOT possible candidate:

#Question id: 4299

#Unit 3. Fundamental Processes

Lactate dehydrogenase (LDH) can exist as various tetrameric isozymes. Extracts from different rat tissues were run on a nondenaturing polyacrylamide gel and stained for LDH activity, as shown below. Standard tetramers of M subunits (M4) and H subunits (H4) were run alongside the tissue extracts. Which of the following is the subunit composition of the isozyme running at position 2 ?

#Question id: 4300

#Unit 3. Fundamental Processes

Find correct order of cotranslational translocation in endoplasmic reticulum

A. Transfer of the nascent polypeptide–ribosome to the translocon leads to opening of this

translocation channel to admit the growing polypeptide adjacent to the signal sequence, which is transferred to a hydrophobic binding site next to the central pore. Both the SRP and SRP receptor, once dissociated from the translocon, hydrolyze their bound GTP and then are ready to initiate the insertion of another polypeptide chain.

B. As the polypeptide chain elongates, it passes through the translocon channel into the ER lumen, where the signal sequence is cleaved by signal peptidase and is rapidly degraded.

C. The SRP and the nascent polypeptide chain–ribosome complex bind to the SRP receptor in

the ER membrane. This interaction is strengthened by the binding of GTP to both the SRP and its receptor.

D. The ER signal sequence emerges from the ribosome; it is bound by a signal recognition particle (SRP).

#Question id: 4301

#Unit 3. Fundamental Processes

Which of the following proteins prevents eukaryotic mRNAs from being recruited to the ribosome prior to egg fertilization