Analysis of the primary structure of the chloroplast isozyme of triosephosphate isomerase from rye leaves by protein and cDNA sequencing indicates a eukaryotic origin of its gene.

The primary structure of the chloroplast isozyme of triosephosphate isomerase from rye leaves was identified by protein and cDNA sequencing and compared to the deduced amino acid sequence of a cDNA for the cytosolic isozyme. The mature cytosolic and chloroplast isozyme proteins share 64% amino acid sequence identity. The cDNA for the chloroplast isozyme codes for a precursor protein consisting of an N-terminal transit peptide of Mr 4351 and a mature subunit of Mr 27,282. Southern blot analysis indicates that the two rye isozymes are encoded by two independent single genes. Amino acid residues or sequence regions of basic functional relevance in known triosephosphate isomerases are strictly conserved in the chloroplast isozyme. The chloroplast isozyme contains 6 cysteine residues, instead of 4 in the cytosolic isozyme. A cysteine at position 143 of the chloroplast isozyme appears to be modified. Phylogenetic trees constructed on the basis of sequence comparisons for triosephosphate isomerases from different species of all major taxonomic groups indicate that the chloroplast isozyme is much more closely related to eukaryotic cytosolic enzymes than to eubacterial enzymes. The results indicate that the nuclear gene for the chloroplast isozyme originated with that for the cytosolic isozyme through duplication of an ancestral eukaryotic gene, rather than through gene transfer from a prokaryotic endosymbiont.