Heterochromatin-based silencing of a foreign tandem repeat in Drosophila melanogaster 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 organismal 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 Drosophila melanogaster that transcripts derived from 1360 and Invader4 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 heterochromatin masses. Here we show that a ~9 kb tandem array of the 36-nucleotide lac operator (lacO) sequence of Escherichia coli can form ectopic heterochromatin at a similar subset of sites, resulting in variegating gene expression of an adjacent reporter gene. HP1a and histone deacetylation are required for lacO repeat-induced silencing, but in contrast with previously described Position Effect Variegation (PEV) we do not observe increased histone H3 lysine 9 methylation. Silencing is suppressed at lower temperatures (another striking contrast to canonical PEV, which is enhanced at lower temperatures), implying the involvement of a temperature-sensitive component. Temperature switching experiments show that lacO repeat-induced heterochromatin formation is reversible throughout larval development following an HP1a-dependent licensing step in the early embryo. We conclude that the Drosophila nucleus can recognize a completely foreign tandem repeat as a target for heterochromatin formation, but find that the heterochromatin structure established is distinct from that of endogenous tandem arrays.