Histone Chaperones Spt6 and FACT: Similarities and Differences in Modes of Action at Transcribed Genes

Figure 1

Proposed models for Spt6 and FACT recruitment to and disengagement from actively transcribing genes. Recruitment (top panel): Several mechanisms for Spt6 recruitment to active genes have been proposed and include direct interactions with the Pol II CTD (tail extending from Pol II in the figure) and indirect interactions with Pol II through either the Paf1 complex (Paf1C in the figure) or Spn1/Iws1 (Spn1 in the figure) serving as bridging factors. The FACT complex is thought to be able to associate indirectly with Pol II with either HP1 or the Paf1 complex bridging the interaction. The chromatin remodeling factor Chd1 and the histone acetyl transferase complex NuA3 can interact with histones and with FACT and likely promote FACT association with chromatin. Disengagement (bottom panel): at some genes, Spt6 and FACT dissociate at distinct locations during the transcription process and, therefore, in these cases each factor must utilize a unique dissociation mechanism (not addressed in this figure). At certain other genes, such as the yeast PMA1 and ADH1 genes, Spt6 and FACT depart chromatin at similar locations. However, the nature of the mechanisms used by the two histone chaperones at this class of genes is likely to be at least to some degree different, with Spt6 using a mechanism that is only modestly sensitive to the H3-L61W mutation and FACT using a mechanism that is very sensitive to the H3-L61W mutation. Green pentagons: Spt6; red pentagons: FACT; thin blue lines: DNA regions flanking the coding region of a transcribed gene; thick blue lines: coding region of a transcribed gene; gray ovals with two blue lines: nucleosomes.