Recognition efficiency of Dicotyledoneae-specific promoter and RNA processing signals in rice

Heterologous gene expression experiments have shown that genes of Monocotyledoneae are often not transcribed in Dicotyledoneae, or produce pre-mRNA that is inefficiently or aberrantly processed. It is however not known how correctly and efficiently dicotyledon-specific gene expression signals are recognized in cells of Monocotyledoneae. Here we address this question using tobacco (Nicotiana tabacum) and rice (Oryza sativa) protoplasts transformed with the same hybrid gene constructs. Constructs including the nptII protein coding sequence fused to Cauliflower Mosaic Virus (CaMV) promoter and polyadenylation signals were used to obtain stably transformed cell lines of tobacco and rice. In one of the constructs the nptII coding region is interrupted by a modified intron-3 sequence from the soybean phaseolin gene. Although the mean number of hybrid gene copies integrated into the rice genome was on average 5- to 10-fold higher than in tobacco, the steady-state transcript level was 3 times lower. A lower level of transcript was also observed in transient expression experiments. The amount of the mature mRNA was not influenced by the presence of the intron. The phaseolin intron was processed in rice with high efficiency and an accuracy indistinguishable from that seen in tobacco.