How does Ssu72 mediate gene looping?
Investigation of chromosome folding in mutants confirms roles for RSC, "gene looping" factor Ssu72, Mediator, H3K56 acetyltransferase Rtt109, and the N-terminal tail of H4 in folding of the yeast genome. The essential N terminus of the Pta1 scaffold protein is required for snoRNA transcription termination and Ssu72 function but is dispensable for pre-mRNA 3'-end processing. TFIIB crosslinks to both the promoter and terminator regions of the PMA1 and BLM10 genes, and its association with the terminator, but not the promoter, is adversely affected by E62K and by depletion of the Ssu72 component of the CPF 3' end processing complex, and is independent of TBP. We propose a model for RNAP II transcription in which promoter and terminator regions are juxtaposed, and that the resulting gene loops facilitate transcription reinitiation by the same molecule of RNAP II in a manner dependent upon Ssu72-mediated CTD dephosphorylation. The first 300 amino acids of Pta1 are sufficient for interactions with Ssu72, which is needed for pre-mRNA cleavage. 