Heterochromatin-based silencing of a foreign tandem repeat in <i>Drosophila melanogaster</i> shows unusual biochemistry and temperature sensitivity

Eukaryotic genomes are packaged into chromatin, a regulatory nucleoprotein assembly. Establishment, maintenance, and interconversion of chromatin states is required for correct patterns of gene expression, genome integrity, and survival. Transcriptionally repressive heterochromatin minimizes mobilization of transposable elements and limits expansion of other repetitive DNA, but mechanisms for recognition of the latter sequences are not well established. We previously demonstrated in <i>Drosophila melanogaster</i> that transcripts derived from <i>1360</i> and <i>Invader4</i> transposon insertions can trigger local conversion of transcriptionally permissive euchromatin to heterochromatin through the piRNA system, but only in a subset of genomic locations near existing blocks of heterochromatin. Here we show that a ~9 kb tandem array of the 36-nucleotide <i>lac</i> operator (<i>lacO</i>) sequence of <i>Escherichia coli</i> can form ectopic heterochromatin at a similar subset of sites, resulting in variegating expression of an adjacent reporter gene. Heterochromatin Protein 1a (HP1a) and histone deacetylation are required for <i>lacO</i> repeat-induced silencing, but, contrasting with previously described Position Effect Variegation (PEV), we do not observe increased histone H3 lysine 9 methylation. Silencing is effective at 25°C and suppressed at 18°C (in contrast to canonical PEV, which is enhanced at 18°C), indicating involvement of a temperature-sensitive component. Temperature switching experiments show that <i>lacO</i> repeat-induced heterochromatin formation is reversible throughout larval development following an HP1a-dependent initiation step in the early embryo. We conclude that the <i>Drosophila</i> nucleus can recognize a completely foreign tandem repeat as a target for heterochromatin formation, and that the heterochromatin structure established is distinct from that of endogenous tandem arrays.