RED ALGAL LHC-I GENES HAVE SIMILARITIES WITH BOTH CHL A B-BINDING ANDA/C-BINDING PROTEINS - A 21 KDA POLYPEPTIDE ENCODED BY LHCAR2 IS ONE OF THE 6 LHC-I POLYPEPTIDES/

Polypeptides from the PS I holocomplex of the red alga P. cruentum, pu rified for microsequencing, confirmed that six LHC I polypeptides from SDS-PAGE are distinct apoproteins. Analysis of a cDNA clone, designat ed as LhcaR2, from a cDNA library, indicates that it shares major stru ctural features with the recently cloned first red algal gene LhcaR1. The LhcaR2 is believed to encode the 21.0 kDa polypeptide of the LHC I complex from comparison of the deduced amino acid sequence and the mi crosequences of several tryptic digest fragments from the isolated pol ypeptide. As in chlorophytic and chromophytic LHCs, the essential resi dues for Chi-binding and helix stabilization in helices 1 and 3 are hi ghly conserved. Relatedness between rhodophytes and the chlorophytes i s also inferred from sequence conservation in the N-flanking regions o f helices 1 and 3. Conversely, helix 2 exhibited the highest similarit y between LHC sequences of Chl a/c-binding chromophytes and the Chi a- binding rhodophytes, with 11 of 22 residues identical or conservativel y substituted. Moreover, whereas in chlorophytes, the Q and E Chl-bind ing residues are separated by seven amino acid residues, they are alwa ys separated by 8 residues in rhodophytes and chromophytes. Superimpos ition of the predicted LhcaR2 sequence with the LHC II model [Kuhlbran dt et al. (1994) Nature 367: 614-621] shows the same structural featur es except shortened connecting sequence between helices 1 and 2 on the lumenal side. The chimeric nature of rhodophyte genes, with both chro mophytic and chlorophytic features, leads to the suggestion that they reflect attributes of an intermediate stage in LHC apoprotein evolutio n.