Question

In: Biology

Explain two main differences between polycistronic and monocistronic m RNAs. 2) What is the catalytic core...

Explain two main differences between polycistronic and monocistronic m RNAs.

2) What is the catalytic core of the ribosome made of and what is the main chemical reaction that is catalyzed there?

It is made up of

It catalyzes

3) Explain one physical mechanism that the ribosome uses to insure that frame shifts don’t happen when aminoacylated – tRNAs enter the A site and base with a codon.

Solutions

Expert Solution

1.

Polycistronic mRNAs Monocistronic mRNAs
1. Codes for more than one protein. 1. Codes for only a single protein.
2. Present in prokaryotes. 2. Present in eukaryotes.

2.

Peptide bond formation and release are essentially catalyzed in the active site of the large subunit of the ribosome where universally conserved nucleotides surround the CCA ends of the peptidyl- and aminoacyl-tRNA substrates The catalytic center in which an inner shell of conserved nucleotides is pivotal for peptide release, while an outer shell is responsible for promoting peptide bond formation.

The RNA is highly organized into various tertiary motifs which show coaxial stacking. The extra RNA in the larger ribosomes in several long continuous insertions, form loops out of the core structure without changing it. All of the catalytic activity of the ribosome is carried out by the RNA; the proteins reside on the surface and seem to stabilize the structure.

3) The ribosome is the most error-prone polymerase with error rates estimated at 10?3 – 10?4 per amino acid incorporated. The most common translational errors are missense or tRNA miscoding errors but non-initiation, stop codon readthrough and mRNA reading frame errors do occur.

Frameshift suppressor tRNASufA6, a derivative of tRNAProCGG, contains an insertion 3’ of the anticodon (after position 37). The absence of a N1-methyl (m1) base modification at G37 in tRNAProCGG is known to cause increased levels of +1 frameshifting. tRNASufA6 stimulates frameshifting on sites with the codon signature CCC-N, and multiple structures of the 70S ribosome were solved with the ASLs of tRNASufA6 decoding the following proline codons (CCC) containing an additional nucleotide decoded as part of the codon: CCC-U, CCC-G and CCC-A. In each case, two Watson-Crick base pairs form between the codon and anticodon, a non Watson-Crick C-C base pair forms at the third (or wobble) position, and no interaction occurs with the downstream nucleotide U/G/A that is part of the four nucleotide codon. These results establish a +1 slip of the reading frame likely occurs after completion of tRNA selection in the A site, either during mRNA-tRNA translocation to the P site, or within the P site itself. These structures also revealed that the U32-A38 base pair, which sits at the top of the anticodon loop, is disrupted in the context of structures containing a +1 frameshift-prone tRNA and corresponding four nucleotide codons. Interestingly, the 32–38 interaction is restored when a +1 frameshift-prone tRNA interacts with a cognate codon that supports 0 frame decoding, signifying both the frameshift suppressor tRNA and a specific four nucleotide codon are required for the frameshift event. Interactions between this region of the ASL, specifically nucleotide 32, with either EF-G during translocation or with the ribosome in the P site suggest that tRNASufA6 may interact with EF-G in a noncanonical manner, resulting in incomplete or slowed translocation. An additional possibility is that normal translocation occurs, but that the 32–38 pairing is not gripped tightly within the P site, which may facilitate movement between the anticodon and codon into a different frame.


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