The effects of kinetin and hydroxyurea on the expression of the endogenous and transgenic Heat Shock Cognate 80 (HSC80)

We previously demonstrated that expression of transgenes under the control of the tomato (Lycopersicon esculentum) Heat Shock Cognate 80 (HSC80) promoter is frequently poor and appears to be silenced. Multiple elements from the HSC80 locus significantly improved transgene expression in Arabidopsis thaliana; however, the promoter alone is sufficient in a transient assay. Treatments that either reactivate silenced transgenes or inactivate active ones may help elucidate requirements for proper regulation. We found that hydroxyurea inhibited HSC80 transgene expression and expression of the corresponding Arabidopsis HSC80 ortholog, HSP81-2. In contrast, kinetin stimulated expression of HSC80 transgenes unless they were either completely silenced or expressed well. Consistent with this observation, kinetin did not stimulate the well-expressed endogenous gene. The opposite effects of a cell-cycle inhibitor, hydroxyurea, and a cell cycle stimulator, kinetin, may be explained by a relationship between DNA replication and HSC80 gene expression.