High-Level and Ubiquitous Expression of the Rice CytochromecGeneOsCc1and Its Promoter Activity in Transgenic Plants Provides a Useful Promoter for Transgenesis of Monocots

Abstract

Expression patterns of a rice (Oryza sativa) cytochrome c gene OsCc1 and its promoter activity were characterized in transgenic rice plants. OsCc1 transcripts accumulate in most cell types, but to varying levels. Large amounts of OsCc1 transcripts are found in the roots, calli, and suspension cells, but relatively lower in mature leaves, demonstrating its higher levels of expression in non-photosynthetic tissues. Unlike the human cytochrome c gene, which is responsive to cAMP, OsCc1 expression is not enhanced in various rice tissues after dibutyryl cAMP treatments. OsCc1 promoter was linked to the sgfp gene and its activities in different tissues and cell types of transgenic rice plants were analyzed in comparison with the Act1 and RbcS promoters. OsCc1 promoter directs expression in virtually all organs of transgenic plants including roots, leaves, calli, embryos, and suspension cells, showing a particularly high activity in calli and roots. Activity of the OsCc1 promoter was 3-fold higher than Act1 in calli and roots and comparable with RbcS in leaves, representing a useful alternative to the maize (Zea mays) Ubi1 and the rice Act1 promoters for transgene expression in monocots.Several factors increase the expression of chimeric genes in transgenic plants. The choice of promoters affects transgene transcription, resulting in changes not only in concentration, but also in the stage, tissue, and cell specificity of its expression. Over the years, several well-characterized promoters have been made available for transgene expression in plants. However, most of these promoters are from dicot plants. Presently, the cauliflower mosaic virus (CaMV) 35S promoter and its derivatives are among the most commonly used. The CaMV 35S promoter is active in monocots, but its relative strength is substantially lower in monocot than in dicot cells. In addition, it is inactive in some cell types; for example, pollen (Bruce et al., 1989;Christensen et al., 1992; McElroy and Brettell, 1994). Other dicot promoters have also been used for monocot transformation, but activity tends to be lower than for monocot promoters (Wilmink et al., 1995). For example, Kyozuka et al. (1993) reported that expression of the gusA gene driven by a tomato (Lycopersicon esculentum) RbcS promoter is induced by light. However, the expression of the rice (Oryza sativa) RbcS::gusA gene is significantly higher than expression of the tomato RbcS::gusA gene. Conversely, the use of monocot promoters has resulted in a high degree of gene expression in monocots, including rice (McElroy et al., 1991;Cornejo et al., 1993; Kyozuka et al., 1993). Several promoters have been investigated as useful alternatives to drive a high level of expression in monocot transgenesis; for example, the rice Act1 promoter (McElroy et al., 1991), the rice RbcS promoter (Kyozuka et al., 1993; Jang et al., 1999), and the maize (Zea mays) Ubi1 promoter (Uchimiya et al., 1993). In many cases, introns strongly enhance transgene expression in transgenic plants (Wilmink et al., 1995). In ...

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