ansmitting neural signals to interneurons in the inner retinal layers, and a unique nuclear chromatin organization to mediate cell-type-specific gene expression while maximizing the amount of light reaching the outer segments. The vast majority of photoreceptors in most mammalian retinas are rods, which are exquisitely sensitive to low levels of light and mediate night vision. 3�C5 of photoreceptors in mouse and human retinas are cones, which mediate color vision in daylight. Cones can be further classified on the basis of the wavelength sensitivity of the light-capturing visual pigment opsin they contain. To establish and maintain their structure and function, each photoreceptor subtype expresses a set of specific genes including the characteristic opsin, under the tight regulation of a network of photoreceptor-specific transcription factors. The central player, the cone-rod homeobox transcription factor CRX, interacts with photoreceptor subtypespecific transcription factors such as NRL and NR2E3 in rods or TRb2 and RXRc in cones, to activate or suppress expression of rod vs. cone gene sets. We demonstrated previously that CRX activates transcription by interacting with coactivators or coactivator complexes including CBP, P300, and GCN5, a component of the STAGA chromatin remodeling complex. All of these coactivators contain intrinsic lysine acetyltransferase activity, catalyzing acetylation of core histone tails and other proteins. Acetylated histones are active marks for transcription, often associated with ����open���� chromatin that is accessible to the transcription machinery and transcription regulators. Histone acetylation is controlled by two classes of enzymes with opposing functions: Histone lysine acetyltransferases add the acetyl groups to specific lysine residues in the tails of core histones, and histone deacetylases remove them. During postnatal mouse retinal development between P10 and P21, post-mitotic Moxisylyte (hydrochloride) opsin-positive photoreceptors undergo terminal differentiation and maturation. At the cellular level, they elaborate outer NCH-51 cost segments containing the phototransduction machinery, and make synaptic connections to inner neurons. At the molecular level, expression of many photoreceptor genes increases to adult levels during this time. All these terminal differentiation processes depend on continuing expression of Ep300 and/or Cbp. Although conditional knockout of both genes causes severe defects, one wild-type allele of either Ep300 or Cbp can mo
