A superfamily ofDrosophilasatellite related (SR) DNA repeats restricted to the X chromosome euchromatin

The 1.688 g/cm3 class of Drosophila satellite DNA is predominantly localized to the centromeric heterochromatin of the X chromosome. We report here the existence of 1.688 satellite related (SR) DNA arrays present at numerous locations throughout the euchromatic portion of the X. Unlike their heterochromatic counterparts, euchromatic SRs consist of a small number of repeating units (usually 2-4), each of which is 63-81% identical to the 359-bp monomer of the 1.688 satellite. Although it appears that SR DNA arrays are not transcribed, in at least two cases, they are located adjacent to transcriptionally active genes. SR sequences also have significant similarity to a previously described Drosophila middle repeat found almost exclusively in the X euchromatin. It seems likely that these X linked sequences are required for sex chromosome specific functions.