Researchers reveal the pioneering role of Zelda in zygotic gene activation

In all animals, early development is controlled by products put into the egg by the mother, and it is not until hours or days later that the zygote takes control of its own development. During this time, the genome of the zygote remains silent as it is reprogrammed to allow for the development of an entirely new organism.

Specialized proteins, termed pioneer transcription factors, drive this reprogramming both in the early embryo and in laboratory cultures during the generation of induced pluripotent stem cells (iPSC). While this pioneer factor-mediated reprogramming is essential for development, much remains unknown about how these proteins function, according to Melissa Harrison, PhD, associate professor of biomolecular chemistry.

Using fruit flies to model this conserved process, Harrison’s team uncovered fundamental features of the protein, Zinc finger early Drosophila activator protein, or Zelda, which is a pioneer transcription factor necessary for preparing the early embryo for development.

“We set out to test whether this essential protein was continuously required to reprogram the genome or whether it was only required during the initial phases and that the process was then carried out by additional, downstream factors,” Harrison said.

The research was recently published in the journal Molecular Cell.

A major challenge, however, was the rapid nature of this early development.

To overcome this challenge, the researchers developed a strategy that uses blue light to rapidly and reversibly inactivate Zelda – a literal light switch to turn “off” or turn “on” Zelda function. Using this novel technique, they showed that Zelda was required throughout the process of genomic reprogramming.

Harrison and her team compared wild-type embryos to those that had the Zelda activator engineered to stop working when exposed to blue light. These experiments demonstrated that once Zelda was inactivated, a host of gene expression ceased.

“This is the first time pioneer-factor activity has been tested so quickly – minutes instead of hours or days – and at this early stage in an early embryo,” Harrison said.

Additionally, in collaboration with Peter Lewis, PhD, an assistant professor of biomolecular chemistry, and Kenneth Zaret, PhD, a professor at the University of Pennsylvania, the team showed that, unlike most transcription factors, Zelda could bind DNA even when it was wrapped around other proteins called histones.

“This sort of success shows the strength of the collaborative nature of UW-Madison,” Harrison said.

Together this work shows that pioneer factors may be required not only during the initial steps of reprogramming, but also for driving sweeping developmental changes, Harrison said.

This essential function may be mediated in part by their ability to bind DNA in nucleosomes providing them broad access to the genome, she said.