NUCLEAR GENES ENCODING CHLOROPLAST HEMOGLOBINS IN THE UNICELLULAR GREEN-ALGA CHLAMYDOMONAS-EUGAMETOS

When the green unicellular alga Chlamydomonas eugametos is grown under light/dark regimes, nuclear genes are periodically activated in respo nse to the changes in light conditions. These genetic responses are de pendent upon the activation of genes associated with photosynthesis (L I616 and LI637), nonphotosynthetic photoreceptors (LI410 and LI818) an d the biological clock (LI818). We report here that the LI410 and LI63 7 genes are part of a small gene family encoding hemoglobins (Hbs) rel ated to those from two unicellular eukaryotes, the ciliated protozoa P aramecium caudatum and Tetrahymena pyriformis, and from the cyanobacte rium Nostoc commune. Investigations of the intracellular localization of C. eugametos Hbs by means of immunogold electron microscopy indicat e that these proteins are predominantly located in the chloroplast, pa rticularly in the pyrenoid and the thylakoid region. To our knowledge, this constitutes the first evidence for the presence of Hbs in chloro plasts. Alignment of the LI637 cDNA nucleotide sequence with its corre sponding genomic sequence indicates that the LI637 gene contains three introns, the positions of which are compared with those in the Hb gen es of plants, animals and the ciliate P. caudatum. Although the LI637 gene possesses a three-intron/four-exon pattern similar to that of pla nt leghemoglobin genes, introns are inserted at different positions. S imilarly the position of the single intron in the P. caudatum gene dif fers from the intron sites in the LI637 gene. The latter observations argue against the current view that all eukaryotic Hbs have evolved fr om a common ancestor having a gene structure identical to that of plan t or animal Hbs.