48. Which protein subunit of RUBICO is coded by chloroplast genome?
(1) Small sub-unit of RUBISCO
(2) Large sub-unit of RUBISCO
(3) Entire RUBISCO
(4) none

The correct answer is (2): Large sub-unit of RUBISCO.

The gene for the large subunit of RUBISCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is encoded by the chloroplast genome (rbcL gene) in higher plants. In contrast, the small subunit is encoded by nuclear genes (rbcS family), and the assembled enzyme consists of both types of subunits—eight large (chloroplast-encoded) and eight small (nuclear-encoded).​

RUBISCO Subunit Genes: Chloroplast vs Nuclear Origin

Introduction

Key phrase: Rubisco large subunit chloroplast genome rbcL gene photosynthesis protein coding

RUBISCO is the most abundant enzyme in green tissues, catalyzing the key step of carbon dioxide fixation in photosynthesis. Its large subunit is universally encoded by the chloroplast genome, while the small subunit comes from nuclear DNA and is imported into the organelle for holoenzyme assembly.​

Explanation of Each Option

• (1) Small sub-unit of RUBISCO

  • Incorrect. The small subunit is encoded by nuclear genes and synthesized in the cytoplasm before import into the chloroplast.​

• (2) Large sub-unit of RUBISCO

  • Correct. The large subunit is encoded by the rbcL gene in the chloroplast genome and synthesized inside the chloroplast stroma.​

• (3) Entire RUBISCO

  • Incorrect. Only the large subunit is encoded by the chloroplast genome; the complete enzyme requires both chloroplast- and nuclear-encoded subunits.​

• (4) None

  • Incorrect. The chloroplast genome retains the gene for the large subunit in all higher plants.​

Key Facts: RUBISCO Genome Coding