Nuclear DNA is normally tightly wrapped around histones rendering the DNA inaccessible to the general transcription machinery and hence this tight association prevents transcription of DNA.  At physiological pH, the phosphate component of the DNA backbone is deprotonated which gives DNA a net negative charge.  Histones are rich in lysine residues which at physiological pH are protonated and therefore positively charged.  The electrostatic attraction between these opposite charges is largely responsible for the tight binding of DNA to histones.
Many coactivator proteins have intrinsic histone acetyltransferase (HAT) catalytic activity or recruit other proteins with this activity to promoters.  These HAT proteins are able to acetylate the amine group in the sidechain of histone lysine residues which makes lysine much less basic, not protonated at physiological pH, and therefore neutralizes the positive charges in the histone proteins. This charge neutralization weakens the binding of DNA to histones causing the DNA to unwind from the histone proteins and thereby significantly increases the rate of transcription of this DNA.
Many corepressors can recruit histone deacetylase (HDAC) enzymes to promoters.  These enzymes catalyze the hydrolysis of acetylated lysine residues restoring the positive charge to histone proteins and hence the tie between histone and DNA.  PELP-1 can act as a transcriptional corepressor for transcription factors in the nuclear receptor family such as glucocorticoid receptors.