The double life of Set1‐MLL in the initiation of transcription

Abstract

The Set1/MLL1‐4 complexes represent the mammalian trithorax family of histone‐lysine methyltransferases. The Set1a/b‐containing complexes are responsible for H3K4 trimethylation, a modification which correlates with both actively transcribed and poised promoters. We show here, using an immobilized template assay, that Set1‐MLL is recruited to a pre‐initiation complex (PIC) on a chromatinized template by the model activator GAL4‐VP16. We observe that this recruitment can occur via a direct interaction with GAL4‐VP16, but is stimulated by the dual presence of the PIC components PAF and Mediator. To further understand the downstream consequences of Set1 function, we assembled synthetically modified H3K4me3 chromatin. The modification stimulated activator‐dependent in vitro transcription. Using a discovery‐based assay involving MuDPIT to identify H3K4‐specific factors captured on the immobilized template, we found that the chromatin remodeler CHD1 is enriched on H3K4me3 templates, but is recruited in an activator‐dependent manner, downstream of Mediator and MLL. In summary, preliminary findings suggest that the initial recruitment of Set1‐MLL is can occur both independent of or cooperatively with a functional PIC, the consequence of which is CHD1 recruitment and the stimulation of transcription. The work described above is supported by a grant from the NIH.