A spatial gradient of expression of a cAMP-regulated prespore cell-type-specific gene in Dictyostelium

Sequence and expression analysis of a cAMP-responsive gene regulated during late development of Dictyostelium discoideum

The 7E gene is expressed late in normal development of Dictyostelium discoideum after pseudoplasmodium formation. After disaggregation of the developing cells, transcription of this gene depends entirely on exogenous 3'5' cyclic AMP (cAMP). The 5' flanking region of the 7E gene contains two TATA box-oligo (dT) promoter motifs but analysis of 7E gene expression by primer extension shows only a single primary transcript with transcription initiating immediately after the most proximal promoter motif during development or in disaggregated cells in the presence of exogenous cAMP. Four C-rich sequences lie within 350bp upstream of the cap site, analogous to the upstream elements implicated in the cAMP regulation of several other Dictyostelium genes expressed in development.

cAMP and cell sorting control the spatial expression of a developmentally essential cell-type-specific ras gene in Dictyostelium

The Dictyostelium ras gene (Dd-ras) is expressed at a low level in vegetative cells, is not expressed between the onset of development and aggregation, and is then re-expressed in the multicellular aggregate stages from the distal, now cAMP-responsive, promoter and from two more proximal promoters. Expression of activated Dd-ras (G12----T12) (Reymond et al. 1986) results in an abnormal developmental phenotype with the formation of aggregates having multiple tips and an inhibition of further development. In this report we investigate the spatial expression of Dd-ras by fusing the 5'-flanking region to the Escherichia coli lacZ gene and by staining aggregates for beta-galactosidase (beta-gal) activity. We show that fusions using 5'-flanking sequences that include all promoters are expressed in approximately 10-20% of the cells randomly scattered within the early aggregate. Our data indicate that these beta-gal-expressing cells migrate to newly formed tips of aggregates and localize in the region that becomes the prestalk zone. Staining is also seen in the very posterior of the organism. The anterior staining appears to be specific for the prestalk A population, and beta-gal activity is subsequently present in stalk cells as developmental proceeds. When only the two more proximal promoters are used to drive lacZ expression, localized staining is seen in the anterior prestalk region, although it is weaker than with the construct carrying all promoters. Moreover, staining is not seen in the posterior domain in the first finger stage, suggesting differences in the spatial expression from the different promoters. Staining is also observed in some cells within the prespore region, which could be anterior-like cells. The pattern of Dd-ras/lacZ staining during tip formation suggests a directed, spiral pattern of cell migration, possibly in response to the proposed spiral gradient of cAMP within the developing aggregate. The pattern of Dd-ras is consistent with the abnormal developmental phenotype caused by expressing an activated Dd-ras Thr12 gene and suggests an essential role for Dd-ras in controlling spatial differentiation.