Lola-I is a promoter pioneer factor that regulates RNA polymerase II pausing during Drosophila embryogenesis

Abstract

Transcriptionally engaged Pol II has been observed at many inactive genes, in a paused state. Since Pol II can be found even in the inactive state, is Pol II recruited to these genes by default? If the recruitment of Pol II is regulated, how is it achieved? If the recruitment is regulated, it is presumably not by transcriptional activators, since Pol II is present even when genes are inactive. Could it be by pioneer factors, factors that regulate chromatin accessibility for the binding of other factors, but do not cause transcriptional activation? In this thesis, I have identified Lola-I, a protein ubiquitously expressed during the late stages of Drosophila embryogenesis, to be necessary for the establishment of paused Pol II at many paused genes. Lola-I acts as a pioneer factor which depletes the promoter nucleosome of paused genes to allow the recruitment of Pol II. Besides, I have characterized in detail the differences between paused and non-paused genes, which are expressed during the late stages of Drosophila embryogenesis. The paused genes are generally broadly expressed and have a basal expression even in inactive tissues. Furthermore, utilizing the Lola-I mutants, I established the role of paused Pol II in decreasing the stochasticity in gene expression. This thesis provides first insights into how paused Pol II occupancy can selectively be established at specific promoters and how this could be utilized to fine-tune gene expression.