We have cloned and sequenced all five members of the gene family for the small subunit (rbcS) of ribulose-1,5-bisphosphate carboxylase/oxygenase from tomato, Lycopersicon esculentum cv. VFNT LA 1221 cherry line. Two of the five genes, designated Rbcs-1 and Rbcs-2, are present as single genes at individual loci. Three genes, designated Rbcs-3A, Rbcs-3B and Rbcs-3C, are organized in a tandem array within 10 kb at a third independent locus. The Rbcs-2 gene contains three introns; all the other members of the tomato gene family contain two introns. The coding sequence of Rbcs-1 differs by 14.0% from that of Rbcs-2 and by 13.3% from that of Rbcs-3 genes. Rbcs-2 shows 10.4% divergence from Rbcs-3. The exon and intron sequences of Rbcs-3A are identical to those of Rbcs-3C, and differ by 1.9% from those of Rbcs-3B. Nucleotide sequence analysis suggests that the five rbcS genes encode four different precursors, and three different mature polypeptides. S1 nuclease mapping of the 5' end of rbcS mRNAs revealed that the mRNA leader sequences vary in length from 8 to 75 nucleotides. Northern analysis using gene-specific oligonucleotide probes from the 3' non-coding region of each gene reveals a four to five-fold difference among the five genes in maximal steady-state mRNA levels in leaves.

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.