In the presence of brassinolide, the activated phosphatase BSU1 dephosphorylates BIN2 and promotes its degradation by the 26S proteasome system, thus blocking its activity. BES1 and BZR1 are then dephosphorylated by PROTEIN PHOSPHATASE2A (PP2A), and the active forms of BES1 and BZR1 move into the nucleus where they regulate the expression of brassinolide response genes

TLS Online TPP Program

#Id: 3972

#Unit 2. Cellular Organization

TLS Online TPP Program

#Id: 3974

#Unit 2. Cellular Organization

E. coli has at least four individual topoisomerases (I through IV). Those of type I (topoisomerases I and III) generally, relax DNA by removing negative supercoils (increasing Lk).

TLS Online TPP Program

#Id: 3976

#Unit 2. Cellular Organization

A bacterial type II enzyme, called either topoisomerase II or DNA gyrase, can introduce negative supercoils (decrease Lk). It uses the energy of ATP to accomplish this.

To alter DNA linking number, type II topoisomerases cleave both strands of a DNA molecule and pass another duplex through the break.

TLS Online TPP Program

#Id: 3977

#Unit 2. Cellular Organization

The degree of supercoiling of bacterial DNA is maintained by regulation of the net activity of topoisomerases, I and II.

TLS Online TPP Program

#Id: 3980

#Unit 2. Cellular Organization

Eukaryotic cells also have type I and type II topoisomerases. The type I enzymes are topoisomerases I and III;

The eukaryotic type II topoisomerases cannot underwind DNA (introduce negative supercoils), but they can relax both positive and negative supercoils.

TLS Online TPP Program

#Id: 3981

#Unit 2. Cellular Organization

The quinolone antibiotics, inhibits bacterial DNA gyrase and topoisomerase IV, as ciprofloxacin (Cipro). The quinolones act by blocking the last step of the topoisomerase reaction, the resealing of the DNA strand breaks. Ciprofloxacin is a wide-spectrum antibiotic.