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

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 Lhcb31, is transcribed in vivo, since it is identical to corresponding sequences in a cDNA clone. The protein deduced from another sequence, Lhcb32, appears as the most divergent type III so far characterized. The partial sequence of a third gene, Lhcb33, was also recovered. The 5' noncoding sequences of Lhcb31 and Lhcb32, in the far upstream region, are characterized by an extremely high AT content and extensive direct repeats. In the near upstream region, two long Lhcb32 segments are very similar to a segment proposed as containing regulatory signals in Lhcb31. Specific binding of nuclear proteins to Lhcb31 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.