Genetics and evolution of chloroplast isozymes of triosephosphate isomerase

Triosephosphate isomerase is an ubiquitous and highly conservative dimeric enzyme, consisting of subunits of Mr 26,000-27,000. Plants usually contain one cytosolic and one plastid isozyme. While 2x wheats also contain one plastid isozyme, 4x wheats contain 3, and 6x cultural wheats contain five plastid isozymes. The multiplicity of the isozyme pattern in 6x wheats is explainable by the presence of three different genomes (AABBDD), each contributing a distinct triosephosphate isomerase gene (alpha',beta,delta), and by the formation of homodimeric and heterodimeric isozyme forms. While the beta beta-form was, as expected, also found in Aegilops speltoides which is regarded as donor of the B genome, the descent of the other genes for plastid triosephosphate isomerase did not occur in accordance with common contentions on the evolution of 6x cultural wheat and its presumptive ancestors. In the reciprocal intergeneric hybrids between wheat and rye, Secalotricum and Triticale, the patterns of both the cytosolic and the plastid-specific triosephosphate isomerases were biparentally inherited, indicating also that the plastid isozyme was nuclear-encoded. Data which are available about amino acid sequences and gene organization and immunological observations show that the cytosolic triosephosphate isomerase of plants is strongly related to other eukaryotic animal triosephosphate isomerase genes. Multiple evidence has been presented that the plastid- specific isozyme represents a distinct polypeptide and is specified by a distinct gene, relative to the cytosolic isozyme. Immunological comparisons indicate that the plastid isozyme shares homologies with the cytosolic isozyme but is not related to the enzyme from prokaryotic cyanobacteria or bacteria. To enable a more precise comparison, plastid triosephosphate isomerase has been cloned from a cDNA library from rye, and cDNA clones are being sequenced. The plastid enzyme of triosephosphate isomerase appears to have evolved from a duplication on an ancestral nuclear gene of the primordial plant cell. For other plastid-specific isozymes evidence exists that their genes were incorporated into the nucleus by gene transfer from a prokaryotic endosymbiont.