Sequential recruitment of HAT and SWI/SNF components to condensed chromatin by VP16

Eukaryotic transcription initiation requires the complex dynamics of hundreds of proteins, many of which are found in large multisubunit complexes [1]. Recent experiments have suggested stepwise recruitment of preassembled complexes, including chromatin remodeling, general transcription factor, mediator, and polymerase complexes [1], in which the actual order of recruitment may vary for different promoters [2]. How do these complexes access-target sequences contained within tightly condensed chromatin? While chromatin remodeling activities may facilitate the accessibility of large transcription and polymerase complexes to promoters, it is not known how they themselves are targeted within condensed chromatin. Gene activation in the context of condensed chromatin does occur. A yeast acidic activator, Gal4, can overcome heterochromatin gene silencing in Drosophila [3], and the addition of LCRs (locus control regions) to transgenes overcomes position effect silencing, even within centromeric chromatin [4]. Here, we directly visualize the recruitment of HAT and SWI/SNF components after tethering the VP16 acidic activation domain within condensed chromatin. A recruitment delay of tens to hundreds of minutes for catalytic HAT subunits and SWI/SNF subunits, relative to other HAT and SWI/SNF components, suggests sequential recruitment/assembly of chromatin remodeling complexes within condensed chromatin.