Targets downstream of Cdk8 in Dictyostelium development

Microarray analysis revealed that the cdk8- strain previously described (cdk8-HL) contained genome duplications. Regeneration of the strain in a background lacking detectable gene duplication generated strains (cdk8-2) with identical defects in growth and early development, but a milder defect in spore generation, suggesting that the severity of this defect depends on the genetic background. The failure of cdk8- cells to aggregate unless rescued by exogenous pulses of cAMP is consistent with a failure to express the catalytic subunit of protein kinase A. However, overexpression of the gene encoding this protein was not sufficient to rescue the defect, suggesting that this is not the only important target for Cdk8 at this stage of development. Proteomic analysis revealed two potential targets for Cdk8 regulation, one regulated post-transcriptionally (4-hydroxyphenylpyruvate dioxygenase (HPD)) and one transcriptionally (short chain dehydrogenase/reductase (SDR1)).This analysis has confirmed the importance of Cdk8 at multiple stages of Dictyostelium development, although the severity of the defect in spore production depends on the genetic background. Potential targets of Cdk8-mediated gene regulation have been identified in Dictyostelium which will allow the mechanism of Cdk8 action and its role in development to be determined.The serine/threonine kinase Cdk8 is a regulator of transcription through its association with the mediator complex [1]. This complex was originally identified as an activity that was required to allow RNA polymerase II to perform regulated transcription in vitro. Purification has revealed it be a large multi-protein complex with varying composition. The presence of a submodule containing Cdk8 and its protein partner cyclin C has been proposed to be a mechanism to regulate mediator activity, responsible for both activation and inhibition of transcription through phosphorylation of either the C terminal domain of RNA polymerase II (CTD) or of gene