The selector genes midline and H15 control ventral leg pattern by both inhibiting Dpp signaling and specifying ventral fate.

mid and H15 encode Tbx20 transcription factors that specify ventral pattern in the Drosophila leg. We find that there are at least two pathways for mid and H15 specification of ventral fate. In the first pathway, mid and H15 negatively regulate Dpp, the dorsal signal in leg development. mid and H15 block the dorsalizing effects of Dpp signaling in the ventral leg. In loss- and gain-of-function experiments in imaginal discs, we show that mid and H15 block the accumulation of phospho-Mad, the activated form of the Drosophila pSmad1/5 homolog. In a second pathway, we find mid and H15 must also directly promote ventral fate because simultaneously blocking Dpp signaling in mid H15 mutants does not rescue the ventral to dorsal transformation in most ventral leg structures. We show that mid and H15 act as transcriptional repressors in ventral leg development. The two genes repress the Dpp target gene Dad, the laterally expressed gene Upd, and the mid VLE enhancer. This repression depends on the eh1 domain, a binding site for the Groucho co-repressor, and is likely direct because Mid localizes to target gene enhancers in PCR-ChIP assays. A mid allele mutant for the repressing domain (eh1), mideh1, was found to be compromised in gain-of-function assays and in rescue of mid H15 loss-of-function. We propose that mid and H15 specify ventral fate through inhibition of Dpp signaling and through coordinating the repression of genes in the ventral leg.

In vitro site selection of a consensus binding site for the Drosophila melanogaster Tbx20 homolog midline.

We employed in vitro site selection to identify a consensus binding sequence for the Drosophila melanogaster Tbx20 T-box transcription factor homolog Midline. We purified a bacterially expressed T-box DNA binding domain of Midline, and used it in four rounds of precipitation and polymerase-chain-reaction based amplification. We cloned and sequenced 54 random oligonucleotides selected by Midline. Electromobility shift-assays confirmed that 27 of these could bind the Midline T-box. Sequence alignment of these 27 clones suggests that Midline binds as a monomer to a consensus sequence that contains an AGGTGT core. Thus, the Midline consensus binding site we define in this study is similar to that defined for vertebrate Tbx20, but differs from a previously reported Midline binding sequence derived through site selection.

Imaging the cellular dynamics of Drosophila Argonaute proteins.

Drosophila melanogaster is used extensively as a model system to uncover genetic and molecular pathways that regulate various cellular activities. There are five members of the Argonaute protein family in Drosophila. Argonautes have been found to be localized to cytoplasmic ribonucleoprotein containing structures in both cultured Drosophila cells and developing embryos. However, in fixed cell preparations some Drosophila Argonaute family proteins co-localize with structures containing known as RNA processing (P) body components while others do not. The ability to image Argonaute family proteins in live Drosophila cells, (both cultured and within developing embryos) allows for accurate genetic dissection of the pathways involved in the assembly, mobility, disassembly, and other dynamic processes of Argonaute-containing bodies. Here we describe a method of rapidly creating vectors for, and assay the activity of, fluorescently tagged Argonaute proteins in cultured Drosophila cells and embryos.

Tinman is a direct activator of midline in the Drosophila dorsal vessel.

Heart development requires a conserved core of transcription factors comprised of Nkx2.5, GATA and T-box family transcription factors. In Drosophila melanogaster, the Nkx2.5 gene tinman acts upstream of many cardiac genes including the Tbx20 homolog midline, a critical regulator of heart development in both flies and vertebrates. By testing genomic fragments containing clusters of consensus Tinman-binding sites, we identified a 4.3 kb fragment 5' of midline that directs reporter expression in all midline-expressing heart cells and a 1.7 kb subfragment that drives reporter expression in mid-expressing heart cells that maintain tin expression. Both fragments direct reporter gene expression in response to tinman in transgenic embryos and in transient transfection assays in Drosophila S2 cells. Mutation of two Tinman binding sites (Tin1 and Tin2) reduces or abolishes cardiac expression in derivatives of the 1.7 kb fragment. We conclude that Tin is a direct regulator of midline in fly heart development.

Gawky is a component of cytoplasmic mRNA processing bodies required for early Drosophila development.

In mammalian cells, the GW182 protein localizes to cytoplasmic bodies implicated in the regulation of messenger RNA (mRNA) stability, translation, and the RNA interference pathway. Many of these functions have also been assigned to analogous yeast cytoplasmic mRNA processing bodies. We have characterized the single Drosophila melanogaster homologue of the human GW182 protein family, which we have named Gawky (GW). Drosophila GW localizes to punctate, cytoplasmic foci in an RNA-dependent manner. Drosophila GW bodies (GWBs) appear to function analogously to human GWBs, as human GW182 colocalizes with GW when expressed in Drosophila cells. The RNA-induced silencing complex component Argonaute2 and orthologues of LSm4 and Xrn1 (Pacman) associated with 5'-3' mRNA degradation localize to some GWBs. Reducing GW activity by mutation or antibody injection during syncytial embryo development leads to abnormal nuclear divisions, demonstrating an early requirement for GWB-mediated cytoplasmic mRNA regulation. This suggests that gw represents a previously unknown member of a small group of genes that need to be expressed zygotically during early embryo development.