MOLECULAR-CLONING OF POLYBROMO, A NUCLEAR-PROTEIN CONTAINING MULTIPLEDOMAINS INCLUDING 5 BROMODOMAINS, A TRUNCATED HMG-BOX, AND 2 REPEATS OF A NOVEL DOMAIN

A number of transcription factors that act as adaptor proteins have be en found to contain an 87 amino acid domain called the bromodomain. In a study to identify and characterise bromodomain proteins expressed i n chicken cells, a novel gene has been isolated which encodes five rep eats of the bromodomain. In addition, the encoded protein, termed poly bromo, contains four other domains: an unusual truncated HMG box, two repeats of a novel domain which we term the BAH domain and a sequence related to a region within the regulatory domain of the DNA cytosine-5 methyltransferase enzyme. Polybromo was found to be related to a yeas t protein U19102 which has two bromo domains, a BAH domain and the DNA methyltransferase-related sequence. Antibodies that were raised again st polybromo were used in confocal microscopy analysis to show that th e 180-kDa polybromo protein is located within the nucleus but excluded from the nucleolus. Gel filtration analysis of nuclear extracts demon strate that polybromo is part of a large complex with a mass of approx imately 2 million dalton.