The nuclear gene sequences encoding RBCS, the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) from several plants show extensive interspecific divergence but little intraspecific divergence, suggesting that these genes are evolving in concert within a genome. In this study, the nucleotide sequences of two tomato (Lycopersicon esculentum) RBCS genes and a cDNA clone containing the entire coding region of a third tomato RBCS gene were determined. The three genes, designated Rbcs-1, Rbcs-2A, and Rbcs-3A, each belong to a different one of the three RBCS loci in the tomato genome. The nucleotide sequence of Rbcs-1 differs from that of Rbcs-2A and Rbcs-3A by 13.9% and 13.1%, respectively. Rbcs-2A and Rbcs-3A differ from each other by 10.7%. A recently published RBCS gene sequence from tobacco (Nicotiana tabacum) [Mazur, B. J. & Chui, C.-F. (1985) Nucleic Acids Res. 13, 2373-2386] differs by 10.6% and 11.3% from Rbcs-2A and Rbcs-3A, respectively, and by 15.0% from Rbcs-1. Thus the tobacco gene seems to be phylogenetically as closely related to the tomato genes Rbcs-2A and Rbcs-3A as the latter two are to each other, and more closely related to them than Rbcs-1 is. However, the mature part of the polypeptide encoded by the tobacco RBCS gene differs by five and six amino acids from the corresponding region in the polypeptides encoded by Rbcs-2A and Rbcs-3A, respectively, while these two tomato RBCS polypeptides differ from each other in the mature part by a single amino acid. Rbcs-1, whose nucleotide sequence shows higher divergence from both the tobacco RBCS gene and Rbcs-2A and Rbcs-3A, encodes a polypeptide whose mature part differs by eight amino acids from the corresponding region in the tobacco polypeptide but only by three and four amino acids from the corresponding regions of Rbcs-2A- and Rbcs-3A-encoded polypeptides, respectively. Thus, it appears that in the tomato selection has maintained near uniformity of the coding information in the portion of the RBCS genes encoding the mature polypeptides.

We have isolated and sequenced two cDNA clones (LESS5 and LESS17) encoding the small subunit of ribulose-1,5-bisphosphate carboxylase of tomato (Lycopersicon esculentum). At the nucleotide level, the protein-coding regions of these genes are 85% conserved, while the untranslated 3' regions are only 55% conserved. Comparison with rbcS genes from other species of Solanaceae suggests that the tomato LESS5 gene, the Nicotiana tabacum NTSS23 gene and the Petunia hybrida SSU8 gene are orthologous members of the rbcS gene family. In addition, the tomato gene LESS17, and the Petunia hybrida gene SSU611, may also be orthologous, since their untranslated 3' regions are related. There is a large difference between the two tomato rbcS genes in the frequency of the CG dinucleotide. This difference may reflect different levels of methylation, and therefore expression, of the tomato genes. Many of the differences involving the CG dinucleotide can be represented as transitions between C and T on the noncoding strand. Such changes are consistent with observations that methylated cytosines are hot-spots for transitions.