Structure and developmental regulation of a wheat gene encoding the major chlorophyll a/b-binding polypeptide

Abstract

A genomic clone for a major chlorophyll a/b-binding polypeptide of the light-harvesting complex has been sequenced from wheat. This gene, whAB1.6, encodes a 70-nucleotide 5'-nontranslated spacer, a 34-amino-acid NH2-terminal extension, i.e., the transit peptide, and a mature coding protein of 232 amino acid residues. The exact molecular weight of the precursor polypeptide is 28,560. The transit peptide is basic and is rich in serines. No intervening sequences are found in this gene. The transcription start site of the whAB1.6 gene occurs at AAAC as determined by S1 nuclease analysis. Putative regulatory sequences occur upstream of the gene at -25 (TTTAAATA) and at -72 (CCAACCA). Northern blots show a single RNA species estimated to be 1,100 nucleotides. Heterogeneity of the RNA population is demonstrated in S1 nuclease analyses with a 5'-end-labeled fragment that extends 191 nucleotides into the mature protein coding sequence. At least seven different transcripts can be recognized. The highest levels of RNA transcribed from the whAB1.6 gene are found in the basal segments of the wheat leaf, whereas other chlorophyll a/b-binding transcripts in the cell show a different pattern of abundance. As a control, we show that roots do not contain chlorophyll a/b-binding RNA. The most abundant RNA species shows an interrupted homology with the whAB1.6 gene at the start of the mature protein coding sequence; another species shows homology beginning at the start of the transit peptide and does not include the nontranslated region. Chlorophyll a/b-binding polypeptides accumulate toward the tip of the leaf as shown by Western blot analysis of total thylakoid proteins.