The cosmid clone, CX16-2D12, was previously localized to the centromeric region of the human X chromosome and shown to lack human X-specific α satellite DNA. A 1.2 kb EcoRI fragment was subcloned from the CX16-2D12 cosmid and was named 2D12/E2. DNA sequencing revealed that this 1,205 bp fragment consisted of approximately five tandemly repeated DNA monomers of 220 bp. DNA sequence homology between the monomers of 2D12/E2 ranged from 72.8% to 78.6%. Interestingly, DNA sequence analysis of the 2D12/E2 clone displayed a change in monomer unit orientation between nucleotide positions 585–586 from a “tail-to-head” arrangement to a “head-to-tail” configuration. This may reflect the existence of at least one inversion within this repetitive DNA array in the centromeric region of the human X chromosome. The DNA consensus sequence derived from a compilation of these 220 bp monomers had approximately 62% DNA sequence similarity to the previously determined γ 8 satellite DNA consensus sequence. Comparison of the 2D12/E2 and γ 8 consensus sequences revealed a 20 bp DNA sequence that was well conserved in both DNA consensus sequences. Slot-blot analysis revealed that this repetitive DNA sequence comprises approximately 0.015% of the human genome, similar to that found with γ 8 satellite DNA. These observations suggest that this satellite DNA clone is derived from a subfamily of γ satellite DNA and is thus designated γ X satellite DNA. When genomic DNA from six unrelated males and two unrelated females was cut with SstI or HpaI and separated by pulsed-field gel electrophoresis, no restriction fragment length polymorphisms were observed for either γ X (2D12/E2) or γ 8 (50E4) probes. Fluorescence in situ hybridization localized the 2D12/E2 clone to the lateral sides of the primary constriction specifically on the human X chromosome.
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