Characterization of a Euglena gene encoding a polyprotein precursor to the light-harvesting chlorophyll a/b-binding protein of photosystem II

Abstract

A 7.4 kb segment of a Euglena nuclear gene encoding a portion of the polyprotein precursor to the light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCPII) has been isolated and sequenced. Nine exons can be assembled into a continuous open reading frame encoding 113 amino acids of the C-terminus of an LHCPII, followed by 4 complete LHCPIIs. The intron-exon junctions required to maintain maximum amino acid sequence homology between the LHCPIIs encoded by the Euglena genomic clone and Arabidopsis LHCPII do not conform to the consensus splice site sequences present in most eukaryotic organisms. Three types of LHCPIIs are contained within the polyprotein precursor. There is greater than 90% sequence identity at both the protein and nucleic acid level within a type while between types, there is less than 65% sequence identity. All Euglena LHCPIIs contain three hydrophobic membrane-spanning domains in the same positions as found in other LHCPIIs. Hybridization of genomic Southern blots with a probe encoding LHCPII suggests that the Euglena genome encodes a large number (30-50) of LHCPIIs. A probe derived from the 3' end of the sequenced genomic clone hybridizes with equal intensity to 2-3 genomic fragments on Southern blots. The probe encoding LHCPII hybridizes to RNAs of 9.5 and 6.6 kb on northern blots of total RNA while the 3'-end probe hybridizes only to the 6.6 kb RNA. The 6.6 kb LHCPII band detected on northern blots appears to be composed of transcription products derived from 2-3 LHCPII genes. Euglena appears to be unique in that a large number of LHCPIIs are encoded by a small number (3-5) of polyprotein genes.