The promoter-terminator of chrysanthemum rbcS1 directs very high expression levels in plants.

Transgenic plants are increasingly used as production platforms for various proteins, yet protein expression levels in the range of the most abundant plant protein, ribulose-1,5-bisphosphate carboxylase have not yet been achieved by nuclear transformation. Suitable gene regulatory 5' and 3' elements are crucial to obtain adequate expression. In this study an abundantly transcribed member (rbcS1) of the ribulose-1,5-bisphosphate carboxylase small-subunit gene family of chrysanthemum (Chrysanthemum morifolium Ramat.) was cloned. The promoter of rbcS1 was found to be homologous to promoters of highly expressed rbcS gene members of the plant families Asteraceae, Fabaceae and Solanaceae. The regulatory 5' and 3' non-translated regions of rbcS1 were engineered to drive heterologous expression of various genes. In chrysanthemum, the homologous rbcS1 cassette resulted in a beta-glucuronidase (gusA) accumulation of, at maximum, 0.88% of total soluble protein (population mean 0.17%). In tobacco (Nicotiana tabacum L.), the gusA expression reached 10% of total soluble protein. The population mean of 2.7% was found to be 7- to 8-fold higher than for the commonly used cauliflower mosaic virus (CaMV) 35S promoter (population mean 0.34%). RbcS1-driven expression of sea anemone equistatin in potato (Solanum tuberosum L.), and potato cystatin in tomato (Lycopersicon esculentum Mill.) yielded maximum levels of 3-7% of total soluble protein. The results demonstrate, that the compact 2-kb rbcS1 expression cassette provides a novel nuclear transformation vector that generates plants with expression levels of up to 10% of total protein.