Spatial and temporal expression of a Polysphondylium spore-specific gene

In the cellular slime mold Polysphondylium spherical masses of cells are pe riodically released from the base of the culminating sorogen. These whorls undergo a morphogenetic transformation from spherical to radial symmetry, m arked by the early emergence of a radially symmetric prepattern on the whor l surface. In previous experiments, morphogenesis was followed by observing prestalk cell markers. Here we describe the isolation and characterization of a spore coat gene whose expression pattern is the negative image of the prestalk pattern. To study the molecular mechanism of sp-45 gene regulatio n, we have cloned and analyzed the sp-45 promoter. Deletion analysis locali zed a single positive regulatory element (PRE) to a 106-bp fragment between positions -246 and -352 of the upstream coding sequence. This fragment can be further divided into a promoter-proximal and promoter-distal PRE and a 29-bp sequence between them. The distal PRE can regulate prespore expressio n when fused to a nonfunctioning basal promoter. The distal PRE contains tw o adjacent essential elements, a Gr box (GTGATATAGTGG) and a TA box (TAATAT ATT). Each element can drive prespore cell-specific reporter gene expressio n independently when incorporated into a nonfunctional promoter. Our result s also show that prespore cell-specific gene expression is solely under pos itive regulation, with no evidence for spore-specific enhancers or cis-acti ng negative regulatory elements. By fusing GFP to the C-terminus of sp-45, we have demonstrated that the graded gene expression of SP45 in the sorogen is regulated by a sequence lying within the sp-45 coding sequence. The tem poral and spatial expression pattern of this protein, taken together with t he prestalk expression pattern, demonstrates unambiguously that the radial symmetries that emerge in the whorl are established by a system of position al coordinates and that cell sorting plays little if any role in this proce ss. (C) 2000 Academic Press.