# DBT JRF 2020 — Question 42

Exam: DBT JRF 2020 · Section: B · Topic: Biochemistry · Difficulty: Easy

## Question

Which of the following amino acids is most likely to disrupt an $\alpha$ -helix?

## Options

- **A.** lysine
- **B.** valine
- **C.** arginine
- **D.** proline

## Answer

Option D

## Explanation

Phytochrome A (phyA) is the most abundant phytochrome in dark-grown (etiolated, skotomorphogenic) seedlings, accumulating to very high levels in darkness (constitutively expressed). Upon light exposure, phyA undergoes photoconversion from Pr (red-absorbing, 660 nm peak) to Pfr (far-red absorbing, 730 nm peak). Pfr-phyA is rapidly ubiquitinated by COP1/SPA E3 ligase and degraded via the 26S proteasome (light-labile). This rapid light-induced degradation is why phyA mediates VLFR (Very Low Fluence Response) and HIR (High Irradiance Response) rather than the reversible R/FR responses mediated by light-stable phyB. phyA is essential for de-etiolation responses (hypocotyl elongation inhibition, cotyledon opening, chloroplast development).

Source: https://mypyqbuddy.com/questions/which-of-the-following-amino-acids-is-most-6a05a8a02adafec470293242