P-TEFb,the Super Elongation Complex and Mediator Regulate a Subset of Non-paused Genes during Early Drosophila Embryo Development

Positive transcription elongation factor b (p-tefb) is a kinase consisting of cdk9 and cyclin t that releases rna polymerase ii (pol ii) into active elongation. it can assemble into a larger super elongation complex (sec) consisting of additional elongation factors. here,we use a mirna-based approach to knock down the maternal contribution of p-tefb and sec components in early drosophila embryos. p-tefb or sec depletion results in loss of cells from the embryo posterior and in cellularization defects. interestingly,the expression of many patterning genes containing promoter-proximal paused pol ii is relatively normal in p-tefb embryos. instead,p-tefb and sec are required for expression of some non-paused,rapidly transcribed genes in pre-cellular embryos,including the cellularization gene serendipity-α. we also demonstrate that another p-tefb regulated gene,terminus,has an essential function in embryo development. similar morphological and gene expression phenotypes were observed upon knock down of mediator subunits,providing in vivo evidence that p-tefb,the sec and mediator collaborate in transcription control. surprisingly,p-tefb depletion does not affect the ratio of pol ii at the promoter versus the 3’ end,despite affecting global pol ii ser2 phosphorylation levels. instead,pol ii occupancy is reduced at p-tefb down-regulated genes. we conclude that a subset of non-paused,pre-cellular genes are among the most susceptible to reduced p-tefb,sec and mediator levels in drosophila embryos. © 2015 dahlberg et al.