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Centric regions of eukaryotic genomes are packaged into heterochromatin, which possesses the ability to spread along the chromosome and silence gene expression. The process of spreading has been challenging to study at the molecular level due to repetitious sequences within centric regions. A heterochromatin protein 1 (HP1) tethering system was developed that generates “ectopic heterochromatin” at sites within euchromatic regions of the Drosophila melanogaster genome. Using this system, we show that HP1 dimerization and the PxVxL interaction platform formed by dimerization of the HP1 chromo shadow domain are necessary for spreading to a downstream reporter gene located 3.7 kb away. Surprisingly, either the HP1 chromo domain or the chromo shadow domain alone is sufficient for spreading and silencing at a downstream reporter gene located 1.9 kb away. Spreading is dependent on at least two H3K9 methyltransferases, with SU(VAR)3-9 playing a greater role at the 3.7-kb reporter and dSETDB1 predominately acting at the 1.9 kb reporter. These data support a model whereby HP1 takes part in multiple mechanisms of silencing and spreading.

The Drosophila exuperantia (exu) gene encodes overlapping sex-specific, germline-dependent mRNAs. In this work, the structural differences between these sex-specific exu mRNAs were determined by sequence analysis of 9 ovary and 10 testis cDNAs. The transformer 2 (tra-2) gene functions in sex determination of female somatic cells through its role in regulating female-specific splicing of doublesex (dsx) RNA. We report here that tra-2 is required in male germ cells for efficient male-specific processing of exu RNA; in the absence of tra-2, X/Y males produce a new mRNA which is processed at its 3' end so that it contains sequences normally specific to the female 3' untranslated region. Although the processing event that requires tra-2 occurs in an untranslated region of the exu transcript, the isolation and characterization of a male-specific exu allele which deletes male 3' untranslated sequence indicate that this processing is biologically significant.

Nuclear history affects the fates of mRNAs in the cytoplasm of cells. Proteins loaded onto mRNAs in the nucleus mark RNAs for subsequent translational regulation, stability, degradation, and subcellular RNA localization. New results show that the Drosophila heterogeneous nuclear ribonucleoprotein (hnRNP) Hrp48 contributes to coordinated RNA localization and translational control in oocytes.