RNA polymerase II terminates after a short distance; small oligonucleotides of 4–5 nt are unstable; crystal structures of these RNA/DNA hybrids are unordered. Only longer hybrids have proper base pairing. The short RNA products are degraded rapidly. The suggestion is that this abortive initiation is a form of promoter proofreading.

TLS Online TPP Program

#Id: 3349

#Unit 3. Fundamental Processes

The AlkB protein is an alpa-ketoglutarate-Fe2+ dependent hydroxylase. It catalyzes the oxidative demethylation of 1-methyladenine and 3-methylcytosine residues

TLS Online TPP Program

#Id: 3350

#Unit 3. Fundamental Processes

Methyltransferase catalyzes the transfer of the methyl group on O6 -methylguanine to a cysteine residue on the enzyme, thereby restoring the normal G in DNA.

TLS Online TPP Program

#Id: 3352

#Unit 3. Fundamental Processes

Photoreactivation directly reverses the formation of pyrimidine dimers that result from ultraviolet irradiation.

TLS Online TPP Program

#Id: 3354

#Unit 3. Fundamental Processes

In photoreactivation, the enzyme DNA photolyase captures energy from light and uses it to break the covalent bonds linking adjacent pyrimidines.

TLS Online TPP Program

#Id: 3359

#Unit 3. Fundamental Processes

Another example of direct reversal is the removal of the methyl group from the methylated base O6 –methylguanine  In this case, a methyltransferase removes the methyl group from the guanine residue by transferring it to one of its own cysteine residues. This is costly to the cell because the methyltransferase is not catalytic; having once accepted a methyl group, it cannot be used again.

TLS Online TPP Program

#Id: 3361

#Unit 3. Fundamental Processes

Photoreactivation; Ultraviolet irradiation causes formation of thymine dimers. Upon exposure to light, DNA photolyase breaks the ring formed between the dimers to restore the two thymine residues.