LHCAR1 OF THE RED ALGA PORPHYRIDIUM-CRUENTUM ENCODES A POLYPEPTIDE OFTHE LHCI COMPLEX WITH 7 POTENTIAL CHLOROPHYLL A-BINDING RESIDUES THATARE CONSERVED IN MOST LHCS

The accessory light-harvesting polypeptides associated with photosyste m I (LHCI)! in Porphyridium cruentum bind chlorophyll a, zeaxanthin an d p-carotene. A cDNA library of P. cruentum was screened with an antis erum specific to the LHCI polypeptides, and an 0.9 kb fragment was ide ntified as coding for an LHCI polypeptide. This cDNA; which we named L hcaR1, has an open reading frame encoding 222 amino acid residues incl uding a putative transit peptide of 28 amino acids. Hydropathy analysi s suggests that there are three transmembrane helices in the mature po lypeptide. Each of the amino acid residues that bind chlorophyll (six residues) and serve in stabilizing the helices in higher-plant LHCs ar e conserved in helices I and 3 of P. cruentum LhcaR1. The N-terminal f lanking regions of these two helices also show high sequence conservat ion with other LHCs. Helix 2 contains a seventh putative chlorophyll-b inding site, but resembles helix 2 of higher-plant LHCs to a lesser de gree. A sequence motif of 11 residues found near the N-terminus and in each of the three helices suggests the possibility that the red algal LhcaR1 derives from a gene duplication. Polypeptides of the expected molecular weight in six other red algae (Achrochaetium, Bangia, Callit hamnion Cyanidium, Polysiphonia, Spermothamnion) were recognized by th e antiserum to P. cruentum LHCI, indicating a wide distribution of LHC I in rhodophytes.