Introduction

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the key enzyme of the Calvin cycle, crucial for carbon fixation in photosynthetic organisms, including red and brown algae. Rubisco consists of large (L) and small (S) subunits encoded by genes housed in distinct cellular genomes, reflecting the evolutionary origin of plastids.

Explanation of Each Option

(1) Large subunit is chloroplast and small subunit in nucleus.

• Correct. In red and brown algae, as with most plants and many algae, the genes for the large subunit (rbcL) are located in the chloroplast genome, while the genes for the small subunit (rbcS) are located in the nuclear genome. Both subunits are synthesized in the respective compartments and then assembled in the chloroplast.

(2) Large subunit in nucleus and small subunit in chloroplast.

• Incorrect. This is the reverse of the actual arrangement: the large subunit gene never resides in the nuclear genome, and the small subunit gene is not located in the chloroplast in these plants.

(3) Both large and small subunits in nucleus.

• Incorrect. This scenario is false for all known eukaryotic photosynthetic organisms; only the small subunit gene is in the nucleus, not both.

(4) Both large and small subunits in chloroplast.

• Incorrect. Both genes are never present exclusively in the chloroplast; this is not observed in red and brown algae or higher plants.

Conclusion

The correct encoding sites for Rubisco subunits in red and brown algae are:

• Large subunit (rbcL): chloroplast genome

• Small subunit (rbcS): nuclear genome

This distribution reflects the evolutionary origin of the chloroplast and the integration of gene expression between nuclear and organellar genomes, a concept fundamental to plant cell biology and genetics.