isoleucyl-tRNA synthetase has a nearby editing pocket (a deep cleft in the enzyme) that allows it to proofread the product of the adenylylation reaction. AMP-valine (as well as adenylylates of other small amino acids, such as alanine) can fit into this editing pocket, where it is hydrolyzed and released as free valine and AMP. In contrast, AMP-isoleucine is too large to enter the editing pocket and is therefore not subject to hydrolysis. As a consequence, isoleucyl-tRNA synthetase discriminates against valine twice: in the initial binding and adenylylation of the amino acid (discriminating by a factor of 100), and then in the editing of the adenylylated amino acid (again discriminating by a factor of 100), for an

overall selectivity of 10,000-fold (i.e., an error rate of 0.01%).

TLS Online TPP Program

#Id: 10231

#Unit 6. System Physiology – Plant

BIN2 is a serine/threonine protein kinase that, in the absence of brassinolide, negatively regulates the closely related transcription factors BES1 and BZR1 by phosphorylation.

TLS Online TPP Program

#Id: 10232

#Unit 6. System Physiology – Plant

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: 10233

#Unit 6. System Physiology – Plant

BES1 & BZR1 give BR response

Phosphorylated BIN2 inhibits BES1 & BZR1 

Phosphorylated BIN2 is degraded by BSU1 

BSK proteins activate BSU1 phosphatase.

BRI1–BAK1-BR complex activates BSK

TLS Online TPP Program

#Id: 10234

#Unit 6. System Physiology – Plant

BRI1–BAK1-BR-→ BSK--→BSU1--→degradation of Phosphorylated BIN2-----→activation of BES1 & BZR1--→BR Response

TLS Online TPP Program

#Id: 10235

#Unit 6. System Physiology – Plant

Brassinosteroids, like gibberellins, suppress photomorphogenesis in the dark.

TLS Online TPP Program

#Id: 10236

#Unit 6. System Physiology – Plant

DE-ETIOLATED2 (DET2) gene, which encodes a brassinosteroid biosynthetic gene. Det2 loss-of-function mutants have reduced levels of brassinosteroids, resulting in a de-etiolated appearance of the seedling even when grown in the dark