USADerived Cofactors

The third class of general cofactors, USA, was initially defined as a crude fraction derived from the Pll 0.85 M KCl fraction of HeLa nuclear extracts (see Fig. 2.1 in the Chapter by Hou and Chiang) able to stimulate activator-dependent transcription (Meisterernst et al., 1991). Upon further fractionation of the USA fraction, several positive cofactors (PCI, PC2, PC3, and PC4) and a negative cofactor (NCI) were identified (Kaiser and Meisterernst, 1996). With ongoing research in the study of USA-derived cofactors, it is remarkable to see that each cofactor has a dual role to function as a coactivator in potentiating activator-dependent transcription and also as a repressor in inhibiting basal transcription when the activator is absent.

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