Fig. 7. Chn can bind to the DC enhancer in vitro, and model for the genetic control of macrochaeta SOP singling out. (A) Scheme of the subfragments of the AS1.4DC enhancer (thin lines under the AS1.4DC thick line) that were assayed for binding by a polypeptide containing the five zinc fingers motifs of Chn (Chn5ZF). A, AvaII; B, BglII; P, PstI; S, SalI: Only the DC6 fragment (in red) was bound by Chn5ZF in an EMSA assay, as shown in (B). A polypeptide with the five zinc fingers motifs of Chn (Chn5ZF) binds to the ³²P-labeled DC6 DNA probe in an EMSA assay. (1) ³²P-labeled DC6 DNA probe alone; (2) labeled DC6 with Chn5ZF present; (3) labeled DC6 with Chn5ZF and an 8-fold molar ratio of cold DC6 added; (4) labeled DC6 with Chn5ZF and an 8-fold molar ratio of cold control DNA (subfragment DC1) added. (C) A 0.5 kb fragment of the DC enhancer (PB0.5DC, panel A) directs lacZ expression only in the posterior DC SOP (García-García et al., 1999). (D) Overexpression of UAS-chn (C765-Gal4) promotes expression in many cells of the posterior notum DC region. (E) A combination of prepattern factors (PFs) acting on an ASC proneural cluster enhancer (PE) activate sc expression. The Dl/N signaling pathway, activated by Sc in the proneural clusters, blocks the SOP-specific enhancer (Artavanis-Tsakonas et al., 1995; Culí and Modolell, 1998; Giagtzoglou et al., 2003). An idealized representation of Sc accumulation in the proneural cluster is shown at the bottom of the panel. (F) Sc activates chn. This activation might be direct and mediated by E-boxes present in the chn gene. Chn binds to the PE and further stimulates sc expression, leading to higher accumulation of this protein, and also of Ac (data not shown). The ac/sc-chn stimulatory loop is established. Dl/N signaling still blocks the SOP-specific enhancer. (G) In a poorly understood process, a cell with high levels of Sc accumulation, and helped by the EGFR signaling pathway that is also activated by Sc (Culí et al., 2001), establishes a Sc self-stimulatory loop that is mediated by the SOP-specific enhancer (Culí and Modolell, 1998; Giagtzoglou et al., 2003). This cell accumulates much Sc, Ac and Sens and becomes the SOP. The Dl/N pathway no longer blocks the SOP enhancer in this cell, but it does so in the neighboring cells (Artavanis-Tsakonas et al., 1995).