High-Level, Seed-Specific Expression of Foreign Coding Sequences in Brassica napus

Abstract

The napin seed storage proteins of oilseed rape (Brassica napus) are encoded by a multigene family of at least 16 members. We have isolated five independent napin genes and five napin cDNAs from a rapid cycling line of B. napus (CrGC45). Two of the isolated genomic fragments, G1 and G2, were introduced into tobacco, and napin gene expression was analysed in the developing seeds. Both genes are expressed during the maturation phase of transformed tobacco seeds as judged by the accumulation of napin RNA in immature embryos. These genes appear to utilise the same transcription start site in tobacco as in Brassica, but are expressed at 20-50-fold lower levels in the heterologous host plant. The G1 and G2 promoter regions and 3' flanking regions were also fused to two foreign coding sequences: the chloramphenicol acetyl transferase (CAT) coding sequence and the pea seed 2S albumin (PSA) coding sequence, and these chimeric genes were introduced into B. napus and tobacco. Upon introduction into tobacco, only the PSA mRNA accumulated to a measurable extent. In transgenic rapeseed plants, PSA RNA accumulated to high levels, but CAT RNA was present only at low concentrations. The results demonstrate that high level expression of foreign coding sequences is possible in developing seeds of B. napus and that differential mRNA stability may contribute significantly to the level of foreign transcripts which can be observed in genetically engineered rapeseed.