Nucleotide Sequence of a cDNA Clone Corresponding to the Maize Globulin-2 Gene

Abstract

Maize globulins represent a group of saline-soluble proteins that accumulate primarily in the developing embryo (4, 6, 7). The most abundant globulin components are the Mr 63,000 GLB1 protein and the Mr 45,000 GLB2 protein, which are encoded by the Globulin-i (Glbl) and Globulin-2 (Glb2) genes, respectively (4, 7). GLB 1 and GLB2 collectively comprise 10 to 20% of the protein present in the mature maize embryo. The amino acid composition and the developmental accumulation pattern of GLB 1 and GLB2 suggest that these are embryo storage proteins (4). GLB1 exhibits sequence similarity to 7S seed globulins of both monocots and dicots (2, 3). A common characteristic of seed storage proteins described to date is that they are encoded by multigene families (8). In contrast, the Glbl gene exists as a single copy in the haploid genome as determined by genetic analyses of several electrophoretic variants (7) and as recently confirmed at the molecular level by gene copy-number reconstruction analysis (2). Although we have not observed electrophoretic variants for GLB2, two null alleles for Glb2 have been identified (Table I). GLB2 also appears to be encoded by a single gene as determined by genetic analyses in which each of the Glb2 null alleles segregates as a Mendelian recessive trait and by Southern blot analysis of maize DNA in which Glb2-specific clones are used as probes. The availability of allelic variants for both of the single-copy maize Glb genes allows for detailed analysis of allelic polymorphism in plants at the molecular level. Recently, we reported (2) the isolation and characterization of a cDNA clone corresponding to the Glb]-S allele, which represents the first monocot 7S globulin to be analyzed at the molecular level. We present here the nucleotide sequence of a full-length cDNA clone corresponding to Glb2. The amino acid sequence deduced from the pcGlb2 nucleotide sequence (Fig. 1) indicates that GLB2 is synthesized as a preproprotein, as is the case for GLB 1 (2, 5) and other seed storage globulins (8). Alignment of mature GLB 1 and GLB2 from their amino termini indicates that the two polypeptides share 28% amino acid sequence identity. Although a slight amount of immunological cross-reactivity has been observed for the GLB2 and GLB1 polypeptides, the two cDNA clones pcGlb2 and pcGlb1S do not cross-hybridize at a Tm of -25?C. Efforts are underway to isolate genomic clones corresponding to Glb2 as well as the null alleles Glb2-0 and Glb2-NJ to determine the molecular basis of the null phenotype in each case.