A NOVEL CAROTENOID BIOSYNTHESIS GENE CODING FOR ZETA-CAROTENE DESATURASE - FUNCTIONAL EXPRESSION, SEQUENCE AND PHYLOGENETIC ORIGIN

A DNA fragment which has been isolated previously from an Anabaena DNA expression library was subcloned. The corresponding protein was overexpressed in Escherichia coli. The recombinant enzyme was fully active in converting zeta-carotene into lycopene in vitro with neurosporene as an intermediate. A smaller fragment which still contained the active enzyme was sequenced. An open reading frame of 1497 bp was found coding for a protein consisting of 499 amino acids with the calculated molecular weight of 56740. In a computer search of nucleotide sequences contained in the EMBL nucleotide sequence library, all the best-fitting comparisons were carotenoid desaturases. The highest similarity was found with the crtI phytoene desaturase genes of bacteria and the al-1 gene from Neurospora crassa. A much lower similarity was found with the pds genes coding for phytoene desaturase from cyanobacteria and higher plants. It is shown in protein similarity plots that the amino acid similarity of zeta-carotene desaturase to the latter is mainly limited to the N terminus of the polypeptides. In contrast, the protein similarity plots and a comparison of a conserved region clearly demonstrate that there is a strong relationship between zeta-carotene desaturase and the phytoene desaturases from various bacteria and fungi. Therefore we propose that the zeta-carotene desaturase gene is homologous to the crt I phytoene desaturase genes of bacteria and fungi.