Multiple conserved 5' elements are required for high-level pollen expression of the Arabidopsis reproductive actin ACT1.

The Arabidopsis thaliana actin gene family comprises eight genes, which are divided into two ancient classes, vegetative and reproductive. We dissected various 5' elements and the conserved expression pattern of the reproductive actin gene ACT1, which is the most strongly expressed pollen actin gene. A basal construct containing only 310 bp of sequence upstream of the major transcriptional start site showed essentially full promoter activity in pollen and ovules. Further truncations of the 5'-flanking region and two different 10 bp replacements within a 55 bp conserved domain each caused a several-fold reduction in mature pollen expression. Intron L, located in the 5'-untranslated region (5'-UTR) was also required for high-level expression of pollen and organ primordia, and it had the properties of an enhancer. Pollen expression was not preserved when intron L was precisely replaced by intron L2 from the vegetatively expressed actin gene ACT2. ACT1 reporter gene constructs were strongly expressed in both pollen and ovules of tobacco and in the pollen of rice. Promoter-reporter fusions of the most distantly related Arabidopsis reproductive actin gene ACT1 showed strong expression in tobacco pollen and ovules indistinguishable from that directed by ACT1. Thus, multiple conserved cis-sequence elements within the 5'-flanking region and 5'-UTR of ACT1 direct high levels of reproductive tissue-specific expression.