A cDNA Clone Encoding a Light-Harvesting Protein from Mantoniella squamata

Abstract

The green algae and land plants are generally assumed to share a common system of harvesting light energy for photosynthesis based on the plastid Chl a/b protein LHCs, which are encoded by the nuclear Cab gene family. Indeed, the polypeptide constituents of most of these complexes are clearly homologous, as demonstrated by sequence analysis (Pichersky and Green, 1990) and immunochemical crossreactivity (Thornber et al., 1991). However, one group of small green flagellates, members of the order Mamiellales of the Micromonadophyceae, have a major LHC that does not display immunochemical cross-reactivity to the major LHC of other green plants (Fawley et al., 1990). The LHC of these organisms also contains the xanthophyll prasinoxanthin and a Chl c-like pigment in addition to Chl b, a combination of pigments not found in land plants or other types of green algae (Fawley, 1992). Some members of the Mamiellales, notably Mantoniella squamata (Manton et Parke) Desikachary, have been considered by some to represent the most primitive type in the green algae (Melkonian, 1984; van den Hoek et al., 1988). Therefore, it was of interest to us to use sequence analysis to determine whether or not the polypeptides of the LHC of the Mamiellales are homologous to those of other green algae and land plants. In addition, the comparison of a highly divergent or primitive LHC type with other LHCs could provide insight on important structural features of the proteins. A cDNA library from poly(A)+ RNA from Mantoniella squamata was constructed using the SuperScript Lambda System (BRL). The library was screened with polyclonal antisera raised against the major polypeptide of a LHC of M. squamata (Lee et al., 1992). A positive clone (MScab-1) was subcloned into pBluescript II for restriction mapping with BamHI, HindIII, KpnI, PstI, Sad, SpnI, and XbaII. Appropriate fragments were sequenced in both directions using M13mpl8 and M13mpl9 vectors and Sequenace 2.0 (United States Biochemical). Sequencing revealed that the 900-bp clone contained an open reading frame corresponding to a 231-amino acid polypeptide with a molecular mass of 25.4 kD. The derived amino