MOLECULAR STUDY OF A LIGHT-HARVESTING APOPROTEIN OF GIRAUDYOPSIS STELLIFER (CHRYSOPHYCEAE)

We have isolated a gene from a library of nuclear DNA for a chlorophyl l a/c-binding protein (named Cac for chi a/c by analogy with Cab for c hi a/b) of a chromophyte alga, Giraudyopsis stellifer, and sequenced i t. The comparison of the deduced amino acid sequence with other chi a/ c- and chi a/b-binding protein sequences shows that structural and fun ctional features, i.e. the arrangement 'en X' of the two A and B trans membrane helices and the putative chi a-binding sites, are shared by b oth Chlorophyta and Chromophyta. Moreover, in contrast to Chlorophyta, a very strong identity is found among Chromophyta in the C helix, sug gesting a major function associated to this specific region. Neverthel ess, the primary structure of the apoprotein does not seem affected by the pigment composition in Chromophyta. As in the few other examples currently known, we confirm that the cac genes are nuclear-encoded and are part of a multigenic family. Northern blots, performed on poly(A) (+) mRNA from G. stellifer, give evidence that the cac gene is light-i nduced at a transcriptional level and that no expression can be observ ed in the dark.