Analysis of the maize polyubiquitin-1 promoter heat shock elements and generation of promoter variants with modified expression characteristics.

The maize polyubiquitin-1 (Ubi-1) promoter is one of a few select promoters used to express foreign genes in monocots, such that recombinant proteins can be produced at commercially viable levels. Modifying the activity, specificity and responsiveness of such promoters provides a means to achieve desired levels and patterns of expression of genes encoding target products. Ubi-1 is constitutively expressed but is further induced by heat shock. The promoter contains two overlapping sequences with similarity to defined heat shock elements and we show that these sequences are also present upstream of the Ubi-1 homologue isolated from teosinte. Both the maize and teosinte promoters can mediate a heat shock response in transgenic maize. We have dissected the overlapping maize Ubi-1 promoter heat shock elements and demonstrate that the 3' element is required to mediate a heat shock response. The Ubi-1 promoter is particularly active in tissues consisting of rapidly dividing cells, and within the seed it is strongly biased towards driving expression in the embryo. However, replacement of the heat shock elements with a trimer of a basic domain/leucine zipper factor binding site of a pea lectin promoter shifts the balance in seed expression towards the endosperm. The Ubi-1 variants described here differ in their overall activity in the seed, but they all show potential for driving high levels of heterologous gene expression in maize.