Evaluation of seed storage protein gene 3'-untranslated regions in enhancing gene expression in transgenic rice seed

3' untranslated regions (UTRs) are important sequence elements that modulate the expression of genes. We evaluated the potential of the 3'-UTRs of 9 seed storage protein (SSP) genes as terminators in enhancing the expression of the β-glucuronidase (gus A) reporter gene driven by the glutelin GluB-3 promoter in stable transgenic rice lines. Six of the 3'-UTRs significantly enhanced the activity of the GluB-3 promoter without changing its tissue specificity but altered its expression pattern in endosperm. With the 3'-UTRs of GluB-5, GluA-2 and GluC, the expression of gus A was higher by 3.12-, 2.45- and 2.14-fold, respectively, than with the Nos terminator. These three 3'-UTRs, combined with GluC, Ubi-1 and CaMV35S promoters, also increased GUS levels in stable transgenic rice lines or in transient expression in protoplasts, which indicated that the enhancements were independent of the promoter sequence. The increase in protein production was accompanied by altered mRNA levels, which suggests that the enhancements were due to increased transcript level. The 3'-UTRs of GluB-5, GluA-2 and GluC, when combined with strong promoters, might be ideal candidates for high production of recombinant proteins in rice seeds. The 9 SSP 3'-UTRs could function as faithful terminators in mono- or multi-gene transformation avoiding homology-based gene silencing.