Nucleotide sequences and unusual electrophoretic behavior of the W chromosome-specific repeating DNA units of the domestic fowl, Gallus gallus domesticus.

Nucleotide sequences of three independently cloned repeating units of the W chromosome-specific repetitive DNA sequences ("XhoI family") of the chicken were determined. All three units are 717 bp long with XhoI sites at both ends. There are only 21 sites out of 717 bases where a single base change occurs in one of the three clones. Each of these repeating units consists of 34 tandem repeats of about 21 bp. Sequences of some members of these internal repeats are not well conserved, but the majority of the repeats are characterized by the presence of (A)3-5 and (T)3-5 clusters separated by 6-7 relatively G + C-rich base pairs. One striking feature of the cloned 717 bp repeating units is that they migrate unusually slowly on electrophoresis in polyacrylamide gels. The same feature is also shown by a genomic population of the 0.7 kb repeating units recovered from XhoI digests of the genomic DNA of the female chicken. This anomalous behavior is attributed to the occurrence of DNA curvatures because of the above sequence characteristics and partial recovery of the electrophoretic mobility in the presence of distamycin A. Another feature of the 717 bp repeating unit is the presence of 438 and 159 nucleotide-long open reading frames (ORFs) at each end of the unit. A possible function of the XhoI family sequences in the heterochromatization of the W chromosome and the significance of the ORFs are discussed.