The highest concentrations of free auxin in the living plant are in the apical meristems of shoots and in young leaves because these are the primary sites of auxin synthesis. 

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.

TLS Online TPP Program

#Id: 3363

#Unit 3. Fundamental Processes

The two chromophores in E. coli photolyase (Mr 54,000), N5 N10-methenyltetrahydrofolylpolyglutamate (MTHF polyGlu) and FADH , perform complementary functions.

TLS Online TPP Program

#Id: 3365

#Unit 3. Fundamental Processes

In reversal, MTHF polyGlu functions as a photoantenna to absorb blue-light photons.

TLS Online TPP Program

#Id: 3368

#Unit 3. Fundamental Processes

In E. coli, DNA Pol IV (DinB) or DNA Pol V (a complex of the proteins UmuC and UmuD0 ) performs translesion synthesis.