The combination of a novel stimulatory element in the first exon of the maize Shrunken-1 gene with the following intron 1 enhances reporter gene expression up to 1000-fold
Both exon 1 and intron 1 of the maize Shrunken-1 (Sh1) gene individually stimulate expression of reporter genes in transient gene expression experiments if present within the transcription unit. The Sh1 exon 1 mediates a 10-fold increase in activity when inserted at the 5′ end of the bacterial chloramphenicol transacetylase (CAT) marker gene in both monocot and dicot protoplasts. The Sh1 intron 1 enhances chimeric gene expression in rice and maize protoplasts approximately 100-fold but inhibits CAT expression in tobacco protoplasts. In combination, the stimulatory effects of Sh1 exon 1 and intron 1 are multiplicative in monocot protoplasts resulting in a final enhancement of up to 1000-fold compared to the unmodified CAT or luciferase marker genes.