Heterochromatin protein 1 binds transgene arrays

Heterochromatin protein 1 (HP1) of Drosophila and its homologs in vertebrat es are key components of constitutive heterochromatin. Here we provide cyto logical evidence for the presence of heterochromatin within a euchromatic c hromosome arm by immunolocalization of HPI to the site of a silenced transg ene repeat array. The amount of HP1 associated with arrays in polytene chro mosomes is correlated with the array size. Inverted transposons within an a rray or increased proximity of an array to blocks of naturally occurring he terochromatin may increase transgene silencing without increasing HP1 label ing. Less dense anti-HP1 labeling is found at transposon arrays in which th ere is no transgene silencing. The results indicate that HPI targets the ch romatin of transposon insertions and binds more densely at a site with repe ated sequences susceptible to heterochromatin formation.