THE CHROME SHADOW DOMAIN, A 2ND CHROME DOMAIN IN HETEROCHROMATIN-BINDING PROTEIN-1, HP1

The chrome domain was originally identified as a protein sequence moti f common to the Drosophila chromatin proteins, Polycomb (Pc) and Heter ochromatin protein 1 [HP1; Pare and Hogness (1991) Proc. Natl, Acad. S ci, USA, 88, 263-267; Pare (1990) Trends Genet., 6, 416-421], Here we describe a second chrome domain-like motif in HP1. Subsequent refined searches identified further examples of this chrome domain variant whi ch all occur in proteins that also have an N-terminally located chrome domain, Due to its relatedness to the chrome domain, and its occurren ce in proteins that also have a classical chrome domain, we call the v ariant the 'chromo shadow domain'. Chrome domain-containing proteins c an therefore be divided into two classes depending on the presence, fo r example in HP1, or absence, for example in Pc, of the chrome shadow domain, We have also found examples of proteins which have two classic al chrome domains, The Schizosaccharomyces pombe SWI6 protein, involve d in repression of the silent mating-type loci, is a member of the chr ome shadow group, The similar modular architecture of SpSWI6, HP1 and HP1-like proteins supports the model that the specificity of action of chromatin proteins is generated by combinations of protein modules.