Connecting the developmental patterning of tissues to the mechanistic control of 
RNA polymerase II remains a long-term goal of developmental biology. Many key 
elements have been identified in the establishment of spatial-temporal control 
of transcription in the early Drosophila embryo, a model system for 
transcriptional regulation. The dorsal-ventral axis of the Drosophila embryo is 
determined by the graded distribution of Dorsal (Dl), a homolog of the nuclear 
factor κB (NF-κB) family of transcriptional activators found in humans [1, 2]. A 
second maternally deposited factor, Zelda (Zld), is uniformly distributed in the 
embryo and is thought to act as a pioneer factor, increasing enhancer 
accessibility for transcription factors, such as Dl [3-9]. Here, we utilized the 
MS2 live imaging system to evaluate the expression of the Dl target gene short 
gastrulation (sog) to better understand how a pioneer factor affects the kinetic 
parameters of transcription. Our experiments indicate that Zld modifies 
probability of activation, the timing of this activation, and the rate at which 
transcription occurs. Our results further show that this effective rate increase 
is due to an increased accumulation of Dl at the site of transcription, 
suggesting that transcription factor "hubs" induced by Zld [10] functionally 
regulate transcription.