Minimal regions in the Arabidopsis PISTILLATA promoter responsive to the APETALA3/PISTILLATA feed back control do not contain a CArG box

PISTILLATA (PI) is a floral homeotic B function gene in Arabidopsis and together with the other B function gene, APETALA3 (AP3), is involved in specifying petal and stamen identities. The expression of PI and AP3 is under similar developmental control. The initiation of AP3 and PI expression is at least partly caused by the floral meristem identity gene LEAFY, but the maintenance of AP3 and PI expression involves an autoregulatory loop requiring the activity of both genes. PI and AP3 are MADS domain proteins that form, and appear to function as, a heterodimer. AP3/PI binds in vitro to a sequence motif, CC(A/T)(6)GG, a MADS domain protein consensus binding site also known as the CArG box. We identified a 481-bp PI promoter region that confers both the initiation and the maintenance of PI expression patterns. We further dissected the promoter and identified minimal regions responsible for the AP3/PI-dependent expression. No CArG box is present in these minimal regions, suggesting that either AP3/PI does not bind directly to the PI promoter for the maintenance control, or that it requires additional factors to bind to the PI promoter. Our results suggest that the mechanisms of regulation of the two B function genes, AP3 and PI, are different, because CArG boxes are present in the AP3 promoter and are necessary for the AP3 feedback control.