Isolation of Lhcb3 sequences from Brassica napus: evidence for conserved genes encoding LHCII type III chlorophyll a/b binding proteins.

Abstract

Three closely related sequences were isolated from Brassica napus genomic DNA and were identified as Lhcb3 (genes encoding type III chlorophyll a/b binding proteins of LHCII, the major light-harvesting complex of photosystem II). These genes, as was observed for a tomato Lhcb3, contain two introns and yield both divergent and conserved predicted amino acid segments as compared with type I and type II polypeptides. One of the B. napus genes, designated Lhcb3*1, is transcribed in vivo, since it is identical to corresponding sequences in a cDNA clone. The protein deduced from another sequence, Lhcb3*2, appears as the most divergent type III so far characterized. The partial sequence of a third gene, Lhcb3*3, was also recovered. The 5' noncoding sequences of Lhcb3*1 and Lhcb3*2, in the far upstream region, are characterized by an extremely high AT content and extensive direct repeats. In the near upstream region, two long Lhcb3*2 segments are very similar to a segment proposed as containing regulatory signals in Lhcb3*1. Specific binding of nuclear proteins to Lhcb3*1 promoter fragments was detected by electrophoretic mobility-shift assays. The evolutionary relationship between genes for type III polypeptides and the other types present in LHCII is discussed.