Expression Signals and Vectors

Abstract

The promoter is the main determinant of the expression pattern in the Constitutive transgenic plant. Constitutive promoters direct expression in all or almost all promoters tissues, independent of developmental or environmental signals. Such promoters are used for expression of selectable markers and in many other applications. The promoter directing the synthesis of the cauliflower mosaic virus 35S RNA is the most frequently used constitutive promoter. It is highly active in almost all tissues of dicot plants and, albeit at reduced levels, in monocot plants. Despite being regarded as constitutive, not all tissues of a transgenic plant will express 35S promoter driven transgenes equally well and expression patterns in different acceptor plants may also differ (Benfey and Chua 1990). All regulatory elements of the 35S promoter are located in about 350 basepairs with a core region of 46 bases fused to an enhancer region containing a variety of different sequence motifs which interact with cellular transcription factors and together confer the strong and uniform expression characteristics. The promoter is about 50-fold more active in transgenic plants than the CaMV 19S promoter or the nopaline synthase (nos) promoter, two other constitutive promoters that have been used (Lawton et al. 1987; Sanders et al. 1987). The nos promoter, like the octopine synthase (ocs) promoter and the mannopine synthase (mas) promoter, is derived from the agrobacterial T-DNA and although these promoters are regarded as constitutive, their activity can be affected by hormones and wounding (e.g., An et al. 1990). Agrobacterial promoters are mainly used in dicots although activity for the nos and mas promoter in monocots has been described (Meijer et al. 1991). A strong constitutive dicot promoter from an Arabidopsis thaliana ubiquitin gene has been described but is apparently not yet used frequently (Norris et al. 1993). The promoter of figwort mosaic virus could also prove to be a useful alternative to the CaMV 35S promoter (Sanger et al. 1990).