tion factor controlling plant development

C.X. SHA01, Y. TAKEDA2, S. HATANO2, M. MATSUOKA2 and H.-Y. Hirano1

1) Graduate School of Agriculture and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan

2) BioScience Center, Nagoya University, Nagoya, 464-0814 Japan

     Plant development is regulated by genes encoding transcriptional regulators, which contain characteristic domains such as homeo and MADS domains. Genes encoding the SBP domain are a new member of transcriptional regulator genes, which include SBPI and SBP2 from Antirrhinum majus (Klein eta!. 1996), ligulessl (Igl) from Zea mays (Moreno et a!. 1997) and SPL3 from Arabidopsis thaliana (Cardon et al. 1997). SBP1 and SBP2 proteins bind the promoter region of the floral meristem identity gene SQUAMOSA and the genes are expressed in early flower development of Antirmhinum. Arabidopsis SPL3 is thought to be involved in the floral transition. Maize igi regulates the formation of ligules and auricles in leaf organogenesis. Thus, the SBP genes should play important roles in plant development.

     We have cloned and characterized two genes encoding the SBP domain (OsSBPI and OsSBP2) from Oryza sativa. Both proteins contained highly homologous region at the N-terminal region but did not show any significant homologies at the C-terminal region. The homologous regions, which correspond the SBP domains, were comprised of 77 amino acids and showed 93.5% identity to each other. Basic amino acids were clustered at the N-terminal region of the domain. Sequence in this basic region resembled the putative nuclear localization signal, suggesting that SBP1 and SBP2 may function in the nuclear as transcriptional regulator.

     We compared amino acid sequence of the SBP domain of six proteins and constructed an evolutionary tree (Fig. 1). OsSBP1 and OsSBP2 were closest among these proteins and formed a dade with LG1 from maize. The alignment of amino acid sequences demonstrated that six cysteine and four histidine residues were completely conserved among these six proteins. The conservation of these characteristic amino acids is found in Zn-finger-like transcription factors, such as the Zn-finger and LIM-finger proteins. The Zn-finger-like protein superfamily is classified into several families with sequence similarity of the domain including the Zn-binding motifs. The SBP domain contains a feature of the Zn-binding motif, i. e. conserved Cys and His, but does not share any other similar sequences with the Zn-binding domains of proteins in this superfamily. Therefore, proteins with the SBP domain are considered to be a new member of transcription factors in the Zn-binding superfamily. Since genes encoding SBP domain have not yet been found in genomes of animals and fungi, the SBP gene family may have arisen and evolved during plant evolution.
 

References
Cardon, G.IL,S. HOhmann, K. Nettesheim, H. Saedler and P. Huijser, 1997. Functional analysis of the Arabidopsis
thaliana SBP-box gene SPL3: a novel gene involved in the floral transition. Plant 1. 12: 367-377.
Klein, J., H. Saedler and P. Huijser, 1996. A new family of DNA binding proteins includes putative transcriptional regulators of the Anrirrhinum majus floral meristem identity gene SQUAMOSA. Mol. Gen. Genet.