Position‐effect variegation in Drosophila: the modifier Su(var)3‐7 is a modular DNA‐binding protein

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Abstract
An increase in the dose of the Su(var)3‐7 locus of Drosophila augments heterochromatin‐promoted variegated silencing. The deduced protein sequence of Su(var)3‐7 reveals seven widely spaced zinc fingers. We found that Su(var)3‐7 has affinity for DNA in vitro and that the minimal protein sequence requirement for DNA binding is any module containing two zinc fingers and the interval between them. As Su(var)3‐7 is a heterochromatin‐associated protein, we tested its affinity for various satellite DNA sequences in vitro. The AATAT and 353‐bp elements have the highest affinity. If affinity for satellite DNAs contributes to the presence of Su(var)3‐7 in heterochromatin, a general affinity for DNA, or sequences yet to be determined, suggests a function in the genomic silencing of position‐effect variegation: expansion of heterochromatin, whether continuous by spreading or discontinuous by pairing with sequence elements scattered through euchromatin, could use the affinity of Su(var)3‐7 for DNA.