Light regulation of plant gene expression by an upstream enhancer-like element

Abstract

Light regulates many varied physiological and developmental phenomena during plant growth and differentiation, including the formation of a photosynthetically competent chloroplast from a proplastid¹. The expression of ribulose 1,5-bisphosphate carboxylase small subunit (rbcS) genes is regulated by light in a development- and tissue-specific manner^2,3. In some plant species, phytochrome has been demonstrated to mediate this response^4–7, and photoregulation of rbcS expression occurs at least in part at the level of transcription^8,9. We have shown previously that a 5′-noncoding fragment (4–973 base pairs (bp) upstream of the messenger RNA cap site) of the pea rbcS ss3.6 gene contains all of the nucleotide sequence information necessary to direct the photoregulated expression of a bacterial chloramphenicol acetyltransferase (cat) gene in tobacco¹⁰. Consistent with these findings, Morelli et al.¹¹ have shown by deletion analysis of a second rbcS gene promoter, that the sequences required for photo-regulated expression of rbcS E9 reside within the 5′-noncoding region. They identified an upstream region of ∼700 bp needed for maximum transcription but not light–dark regulation, and a region from −35 to −2 bp which included the TATA box and contained the necessary information for light responsiveness. We now demonstrate that regulatory sequences 5′ distal to the rbcS ss3.6 TATA box and transcriptional start site not only contain the information necessary for maximum expression, but also confer photoregulation. These upstream regulatory sequences function independently of orientation when fused to their homologous promoter or a heterologous promoter.