Intron-exon structure and gene copy number of a gene encoding for a membrane-intrinsic light-harvesting polypeptide of the red alga Galdieria sulphuraria

Genes for light-harvesting proteins (lhc genes) of higher plants are well e xamined. However, little is known about the corresponding genes of algae, a lthough this knowledge might give valuable information about the evolution of photosynthetic antennae. In the case of rhodophytes only two cDNA sequen ces from a single organism, Porphyridium cruentum, have been published. Her e we describe an additional sequence from another species, the thermo-acido philic red alga Galdieria sulphuraria. For the first time also a genomic se quence for a red algal lhc gene is presented. From a cDNA library of G. sul phuraria we isolated a clone containing an open reading frame for a protein of 302 amino acids with a deduced molecular mass of 33.86 kDa. It shares m ajor structural features with eukaryotic light-harvesting polypeptides. A p roposed cleavage site between transit peptide and mature protein gives rise to a transit peptide of 119 amino acids and a mature protein of 183 residu es. Hydropathy analysis suggests that the mature protein consists of three transmembrane helices. Several amino acid residues supposed to bind chlorop hyll a and chlorophyll b in higher plants are conserved. The protein shows up to 69% identity and 81% similarity to the Porphyridium polypeptides in t he transmembrane helices 1 and 3. Using oligonucleotides annealing in the r egions of the start and stop codons of the gene as primers, a DNA sequence was amplified from nuclear G. sulphuraria DNA by PCR. Compared with the cDN A clone, this sequence contains five additional intervening DNA strings of 50-74 bp length. Four of them show typical features of spliceosomal introns with GT-AG borders, and the fifth differs by starting with GC. Three of th e supposed introns are located in similar positions as introns of higher pl ant light-harvesting proteins. Southern blotting and hybridization experime nts indicate that G, sulphuraria contains at least three copies of this gen e. (C) 2000 Elsevier Science B.V. All rights reserved.