Phospholipase C and two inositol polyphosphate (IP) kinases constitute a signaling pathway that regulates nuclear messenger RNA export through production of inositol hexakisphosphate (IP6). The inositol 1,4,5-trisphosphate kinase of this pathway in Saccharomyces cerevisiae, designated Ipk2, was found to be identical to Arg82, a regulator of the transcriptional complex ArgR-Mcm1. Synthesis of inositol 1,4,5,6-tetrakisphosphate, but not IP6, was required for gene regulation through ArgR-Mcm1. Thus, the phospholipase C pathway produces multiple IP messengers that modulate distinct nuclear processes. The results reveal a direct mechanism by which activation of IP signaling may control gene expression.

The chlamyopsin gene (cop) encodes the most abundant eyespot protein in the unicellular green alga Chlamydomonas reinhardtii. This opsin-related protein (COP) binds retinal and was thought to be the photoreceptor controlling photomovement responses via a set of photoreceptor currents. Unfortunately, opsin-deficient mutants are not available and targeted disruption of non-selectable nuclear genes is not yet possible in any green alga. Here we show that intron-containing gene fragments directly linked to their intron-less antisense counterpart provide efficient post-transcriptional gene silencing (PTGS) in C. reinhardtii, thus allowing an efficient reduction of a specific gene product in a green alga. In opsin-deprived transformants, flash-induced photoreceptor currents (PC) are left unchanged. Moreover, photophobic responses as studied by motion analysis and phototaxis tested in a light-scattering assay were indistinguishable from the responses of untransformed wild-type cells. We conclude that phototaxis and photophobic responses in C. reinhardtii are triggered by an as yet unidentified rhodopsin species.