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First published online 16 August 2006
doi: 10.1242/dev.02525

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Fork head and Sage maintain a uniform and patent salivary gland lumen through regulation of two downstream target genes, PH4SG1 and PH4SG2

Elliott W. Abrams^*, Whitney K. Mihoulides and Deborah J. Andrew

Department of Cell Biology, The Johns Hopkins University School of Medicine, 725 N. Wolfe Street, Baltimore, MD 21205-2196, USA.

Author for correspondence (e-mail: dandrew{at}jhmi.edu)

Accepted 7 July 2006

(Fkh) is required to block salivary gland apoptosis, internalize salivary gland precursors, prevent expression of duct genes in secretory cells and maintain expression of CrebA, which is required for elevated secretory function. Here, we characterize two new Fkh-dependent genes: PH4SG1 and PH4SG2. We show through in vitro DNA-binding studies and in vivo expression assays that Fkh cooperates with the salivary gland-specific bHLH protein Sage to directly regulate expression of PH4SG2, as well as sage itself, and to indirectly regulate expression of PH4SG1. PH4SG1 and PH4SG2 encode -subunits of resident ER enzymes that hydroxylate prolines in collagen and other secreted proteins. We demonstrate that salivary gland secretions are altered in embryos missing function of PH4SG1 and PH4SG2; secretory content is reduced and shows increased electron density by TEM. Interestingly, the altered secretory content results in regions of tube dilation and constriction, with intermittent tube closure. The regulation studies and phenotypic characterization of PH4SG1 and PH4SG2 link Fkh, which initiates tube formation, to the maintenance of an open and uniformly sized secretory tube.

Key words: Drosophila, Fork head (Fkh), Prolyl-4-hydroxylase, Sage, Salivary gland, Tube morphogenesis

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