COOPERATIVE DNA-BINDING BY BICOID PROVIDES A MECHANISM FOR THRESHOLD-DEPENDENT GENE ACTIVATION IN THE DROSOPHILA EMBRYO

The Bicoid morphogen directs pattern formation along the anterior-post erior (A-P) axis of the Drosophila embryo. Bicoid is distributed in a concentration gradient that decreases exponentially from the anterior pole, however, it transcribes target genes such as hunchback in a step -function-like pattern; the expression domain is uniform and has a sha rply defined posterior boundary. A 'gradient-affinity' model proposed to explain Bicoid action states that (i) cooperative gene activation b y Bicoid generates the sharp on/off switch for target gene transcripti on and (ii) target genes with different affinities for Bicoid are expr essed at different positions along the A-P axis. Using an in vivo yeas t assay and in vitro methods, we show that Bicoid binds DNA with pairw ise cooperativity; Bicoid bound to a strong site helps Bicoid bind to a weak site. These results support the first aspect of the model, prov iding a mechanism by which Bicoid generates sharp boundaries of gene e xpression. However, contrary to the second aspect of the model, we fin d no significant difference between the affinity of Bicoid for the ant erior gene hunchback and the posterior gene knirps, We propose, instea d, that the arrangement of Bicoids bound to the target gene presents a unique signature to the transcription machinery that, in combination with overall affinity, regulates the extent of gene transcription alon g the A-P axis.