Highly repeated DNA sequence limited to knob heterochromatin in maize

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Abstract
A highly repeated DNA sequence was isolated from the maize genome as a satellite in actinomycin D/CsCl gradients. By using maize stocks differing in their heterochromatin content it was established that the sequence is a major constituent of 1 class of heterochromatin: knob heterochromatin, which can occur at 23 locations in the chromosome complement. The repeating unit, of 185 base pairs, was cloned in plasmid pBR322 [Escherichia coli] and its nucleotide sequence was determined. The presence of this DNA sequence in knob heterochromatin and its absence from centromeric, nucleolar and B chromosome heterochromatin parallels the cytogenetic differentiation previously described for these classes of heterochromatin in maize. Because knob heterochromatin has a distinctive cytological appearance and is unique in showing neocentric activity at meiosis, its association with a particular repeated DNA sequence may reflect a functional role for the sequence in the cell cycle.