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H.M. El–Hodiri, S. Nekkalapudi, L.E. Kelly, S. Bertuzzi; The Retinal Homeobox (Rx) Gene Product Acts as a Transcriptional Activator in Cooperation With Other Retinal Transcription Factors . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3133.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The retinal homeobox (Rx) gene product is necessary for eye specification and development. Rx interacts with the PCE–1 (photoreceptor conserved element–1) sequence common to many photoreceptor–specific promoters. Rx has been reported to function as a weak transcriptional activator in assays using artificial reporter gene constructs containing multiple PCE–1 elements. Additionally, the PCE–1 site has been shown to interact with other homeobox proteins such as QRX/RaxL and Erx. Here we explore the ability of Rx and other PCE–1 binding proteins to cooperate with additional photoreceptor–specific transcription factors to modulate promoter activity. Methods: Xenopus opsin promoter – luciferase (XOP–Luc) DNA and RNAs encoding candidate transcriptional regulators were microinjected into two–cell Xenopus laevis embryos. Luciferase activity was measured in lysates prepared from injected mid–gastrula embryos. Mobility shift assays (EMSA) were performed using in vitro translated proteins, radiolabeled PCE–1 probe, and oligonucleotide competitors. Expression patterns were analyzed by in situ hybridization using whole or sectioned embryos and digoxigenin–labelled antisense riboprobes. Results:We have found that Rx can activate a XOP–Luc reporter construct. We have been investigating a related gene product, RxL, similar to QRX/RaxL. RxL can also bind the PCE–1 element and activate XOP–Luc. Rx and RxL can cooperate with otx5b and Xlmaf, Xenopus analogs of mammalian crx and Nrl, to activate XOP–Luc. Additionally, we have investigated regulation of XOP by other PCE–1–binding proteins. The ventral anterior homeobox (Vax) genes encode proteins with homeodomains that are essentially identical to the PCE–1 binding protein Erx. The three Vax genes identified in Xenopus are expressed in the optic nerve. Additionally, Vax2 and Vax3 are expressed in the ventral retina. Each Vax gene product can bind the PCE–1 site and cooperate with otx5b and Xlmaf to regulate XOP–Luc. Taken together, these results suggest a complex role for PCE–1 binding proteins in the regulation of photoreceptor gene promoter activity in combination with factors such as otx5b/crx and Xlmaf/Nrl.
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