Distinct effects of CDK8 module subunits on cellular growth and proliferation in Drosophila
The Mediator complex plays a pivotal role in facilitating RNA polymerase II-dependent transcription in eukaryotes. Within this complex, the CDK8 kinase module (CKM), comprising CDK8, Cyclin C (CycC), Med12, and Med13, serves as a dissociable subcomplex that modulates the activity of the small Mediator complex. Genetic studies in Drosophila have revealed distinct phenotypes associated with mutations in CKM subunits, yet the underlying mechanism has remained unknown. Using Drosophila as a model, we generated transgenic strains to individually or simultaneously deplete the four CKM subunits in all possible combinations, uncovering unique phenotypes in the eyes and wings. Depletion of CDK8-CycC enhanced E2F1 target gene expression and promoted cell-cycle progression, whereas Med12-Med13 depletion had no significant impact on these processes. Conversely, depleting Med12-Med13 altered the expression of ribosomal protein genes and fibrillarin, reduced nascent protein synthesis, indicating a severe reduction in ribosome biogenesis and cellular growth compared to the loss of CDK8-CycC. These findings reveal distinct in vivo roles for CKM subunits, with Med12-Med13 disruption having a more pronounced effect on ribosome biogenesis and protein synthesis . Overall design: To investigate how different CKM subunits affect cell proliferation and tissue size in the same tissue and developmental stage, we performed single-cell RNA sequencing (scRNA-seq) on wing discs with depleted CKM subunits (CDK8-CycC, Med12, or CDK8-CycC-Med12) using the 10x Genomics platform. Control samples were ap-Gal4>GFP heterozygous wing discs.