The Drosophila morphogenetic protein Bicoid binds DNA cooperatively

Abstract

The Drosophila morphogenetic protein Bicoid, encoded by the maternal gene bicoid, is required for the development of the anterior structures in the embryo. Bicoid, a transcriptional activator containing a homeodomain, is distributed in an anterior-to-posterior gradient in the embryo. In response to this gradient, the zygotic gene hunchback is expressed uniformly in the anterior half of the embryo in a nearly all-or-none manner. In this report we demonstrate that a recombinant Bicoid protein binds cooperatively to its sites within a hunchback enhancer element. A less than 4-fold increase in Bicoid concentration is sufficient to achieve an unbound/bound transition in DNA binding. Using various biochemical and genetic methods we further demonstrate that Bicoid molecules can interact with each other. Our results are consistent with previous studies performed in the embryo, and they suggest that one mechanism to achieve a sharp on/off switch of gene expression in response to a morphogenetic gradient is cooperative DNA binding facilitated by protein-protein interaction.

Finding the Center Reliably: Robust Patterns of Developmental Gene Expression

“…Only active Bcd (unbound to co-repressor) binds to the hb control region on the DNA to activate its transcription. Experiments suggest that this is a cooperative process [11], which we model with a Hill function with a coefficient of 3 [11]. However, the precise form of the activation function is not very important: taking other functional forms or changing the Hill coefficient to unity, for instance, does not alter our results, as the concentration of active Bcd is reduced to very low values at mid-embryo due to the co-repressor (see also below).…”

The Role of Input Noise in Transcriptional Regulation

“…clei at this stage of development (∼ 140 µm 3 ; see Refs [33,35]), this is a concentration of 8 − 48 nM. Although we don't have independent measurements of the absolute Hunchback concentration, this is reasonable for transcription factors, which typically act in the nanoMolar range [36,37,38,39,40,41], and can be compared with the maximal nuclear concentration of Bcd, which is 55 ± 3 nM [33]. Larger burst sizes would predict larger maximal expression levels, or conversely measurements of absolute expression levels might give suggestions about the burst size for translation in the early Drosophila embryo.…”

Solution Structure of the K50 Class Homeodomain PITX2 Bound to DNA and Implications for Mutations That Cause Rieger Syndrome,

“…Another issue concerning the Engrailed Q50K mutant is the observation that it binds to the consensus TAATCC site with an unusually high affinity, which approaches the picomolar range (9). There is no evidence that natural K50 class homeodomains have such a high affinity for DNA (25,26). The full-length PITX2 protein has a K D of 50 nM (25).…”