Fifteen cDNA clones for the small subunit of ribulose-1,5-bisphosphate carboxylase from rice were grouped into two classes based on restriction enzyme maps. One cDNA insert hybridized to a few DNA fragments from a restriction digest of rice genomic DNA. The results indicate that there are different kinds of mRNAs for the small subunit of this enzyme. Two sequences belonging to different classes were determined and compared. Both cDNA sequences contained a 528-bp open reading frame capable of coding for a polypeptide with 175 amino acids. The coding sequences share 91% homology and the deduced amino acid sequences of the precursors to the SSu are 95% homologous. The homology between sequences for the mature protein region is extremely high (99%), and only one amino acid exchange occurs. Comparison with the structures of small subunits from other plants revealed that the small subunit from rice is similar to that from wheat. Extreme codon bias was detected in the cDNAs, and similar codon preferences can be identified for the small subunit and the light harvesting Chi a/b binding protein from other monocot plants, but not from dicot plants. The results suggest that preferential codon usage occurs in the case of genes expressed at high levels on monocot leaves.
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Matsuoka et al. (1988) studied this question.