Isolation of a gene from maize encoding a chlorophyll a/b-binding protein

Abstract

The chlorophyll a/b (CAB) proteins are major components of the light-harvesting complex of photosystems I and II in the thylakoid membranes of chloroplasts. The complexity of these peptides and the structure and relationship of distinct genes encoding them has been studied in detail in tomato, soya bean and Petunia [ 1, 2, 3 ]. This has led to the description of at least 5 classes of CAB genes in tomato based on differences between peptides found within and between PSI and PSII [ 8 ]. The different classes of CAB genes can be subdivided using another criterion, namely the presence of introns: type I genes do not contain introns whereas type I1 genes contain an intron in their coding sequence. One of the major features of CAB genes is their transcriptional regulation by light [ 10]. This leads to a major induction of CAB mRNA in lightgrown green tissues. In maize a CAB gene has been isolated which may represent up to 2~o of the mRNA in light-grown leaves [9]. However, this gene is also expressed to high levels in the dark. Here we describe the isolation of a structurally distinct maize gene encoding CAB. We have constructed a partial Sau3A genomic library of maize line BE10 (Garst Seed Company, Iowa). This library, 3 × l0 s primary clones, was screened with oligonucleotides complementary to wheat CAB cDNA [5] and the insert of a cucumber CAB cDNA clone [4]. This has permitted the isolation of one genomic clone, gCAB48, containing a 13 kb insert. This clone contained a 2.8 kb PstI fragment which hybridised to the above probes. This fragment was subcloned into pUC19 and the complete sequence determined. This revealed that a complete CAB gene including extensive 5' upstream sequence was contained on this Pst I fragment (Fig. 1). The features of this gene are: 1. The gCAB48 gene does not contain an intron. Therefore, it can be classified as CAB type I. Thus, it is of the same type as the previously published maize gene [9]. 2. The predicted pre-CAB polypeptide sequence consists of 264 amino acids and has a molecular mass of 28 240 kDa. 3. Alignment of gCAB48-encoded protein sequence with other monocotyledonous CAB