contributed equally to this work Mammalian TIF1α and TIF1β (KAP-1/KRIP-1) are related transcriptional intermediary factors that possess intrinsic silencing activity. TIF1α is believed to be a euchromatic target for liganded nuclear receptors, while TIF1β may serve as a co-repressor for the large family of KRAB domain-containing zinc finger proteins. Here, we report an association of TIF1β with both heterochromatin and euchromatin in interphase nuclei. Co-immunoprecipitation of nuclear extracts shows that endogenous TIF1β, but not TIF1α, is associated with members of the heterochromatin protein 1 (HP1) family. However, in vitro, both TIF1α and TIF1β interact with and phosphorylate the HP1 proteins. This interaction involves a conserved amino acid motif, which is critical for the silencing activity of TIF1β but not TIF1α. We further show that trichostatin A, an inhibitor of histone deacetylases, can interfere with both TIF1 and HP1 silencing. The silencing activity of TIF1α appears to result chiefly from histone deacetylation, whereas that of TIF1β may be mediated via both HP1 binding and histone deacetylation.

The GATA factor Pannier activates the achaete-scute (ASC) proneural complex through enhancer binding and provides positional information for sensory bristle patterning in Drosophila. Chip was previously identified as a cofactor of the dorsal selector Apterous, and we show here that both Apterous and Chip also regulate ASC expression. Chip cooperates with Pannier in bridging the GATA factor with the HLH Ac/Sc and Daughterless proteins to allow enhancer-promoter interactions, leading to activation of the proneural genes, whereas Apterous antagonizes Pannier function. Within the Pannier domain of expression, Pannier and Apterous may compete for binding to their common Chip cofactor, and the accurate stoichiometry between these three proteins is essential for both proneural prepattern and compartmentalization of the thorax.

The esterase S gene (estS) of Drosophila virilis is specifically expressed in the ejaculatory bulbs of males. Its sequencing shows similarities between estS product and other esterases of different origin. The transcription of estS in ejaculatory bulbs is at least 2 orders of magnitude higher than in other tissues of males. Two promoters, P1 (distal) and P2 (proximal), and two different transcripts were identified. The promoter P2 is used much more efficiently, and in a stringent, tissue-specific manner. The transcription from P1 takes place in different tissues and stages of development of D. virilis. However, the mRNA transcribed from P1 seems to be inactive in translation as there are three open-reading frames (ORF) between P1 and P2, which may block the translation in P1 initiated mRNA. Insertion of sequence containing the three ORFs into the 5' untranslated region of the CAT gene strongly inhibited expression of CAT. Point mutations destroying the three ORFs completely eliminate the inhibitory effect. Hence tissue-specific expression of the estS gene may depend on control at the level of transcription, promoter selection and translation.