W-HETEROCHROMATIN OF CHICKEN - ITS UNUSUAL DNA COMPONENTS, LATE REPLICATION, AND CHROMATIN STRUCTURE

About 65% of DNA in the chicken W chromosome has been shown to consist of XhoI and EcoRI family repetitive sequences. These sequences showed remarkable delay in the electrophoretic mobility at low temperature o n a polyacrylamide gel. Three dimensional structures of the 0.7-kb Xho I and the 1.2-kb EcoRI family repeating units were estimated to be irr egular solenoids using a computer program based on wedge angles of all the 16 dinucleotide steps. Fluorescence in situ hybridization demonst rated that these two family sequences were localized in a major hetero chromatic body in, an interphase nucleus. Incorporation of bromodeoxyu ridine into the W chromosome in the synchronous culture of MSB-1 cells occurred about 1 h later than the peak of S phase. The chromatin stru cture formed along XhoI and EcoRI family sequences was suggested to be different from the total chromatin or chromatin containing the beta-a ctin gene sequence in that the linker DNA lengths of the former were s ignificantly longer. Fractionation of the HaeIII-digested MSB-1 nuclei yielded a chromatin fraction in which XhoI family sequences were part ially enriched. Several DNA-binding proteins showing higher affinity f or the XhoI family sequence were present in this fraction.