Rif1 prolongs the embryonic S phase at the Drosophila mid-blastula transition

Abstract

In preparation for dramatic morphogenetic events of gastrulation, rapid embryonic cell cycles slow at the mid-blastula transition (MBT). In Drosophila melanogaster embryos, down-regulation of cyclin-dependent kinase 1 (Cdk1) activity initiates this slowing by delaying replication of heterochromatic satellite sequences and extending S phase. We found that Cdk1 activity inhibited the chromatin association of Rap1 interacting factor 1 (Rif1), a candidate repressor of replication. Furthermore, Rif1 bound selectively to satellite sequences following Cdk1 down-regulation at the MBT. In the next S phase, Rif1 dissociated from different satellites in an orderly schedule that anticipated their replication. Rif1 lacking potential phosphorylation sites failed to dissociate and dominantly prevented completion of replication. Loss of Rif1 in mutant embryos shortened the post-MBT S phase and rescued embryonic cell cycles disrupted by depletion of the S phase–promoting kinase, cell division cycle 7 (Cdc7). Our work shows that Rif1 and S phase kinases compose a replication timer controlling first the developmental onset of late replication and then the precise schedule of replication within S phase. In addition, we describe how onset of late replication fits into the progressive maturation of heterochromatin during development.

Replication timing in Drosophila and its peculiarities in polytene chromosomes

“…The Rif1 protein, a candidate repressor of replication, is another important factor involved in the regulation of replication timing in D. melanogaster. Rif1 recruits phosphatase PP1 to multiple chromosomal sites, where the latter can dephosphorylate replicative helicase and block premature replication of heterochromatin sequences (Sreesankar et al, 2015;Seller, O'Farrell, 2018). During amplification of chorion genes in ovarian follicle cells, Rif1 is localized in active replication forks in a partially SUUR-dependent manner, where it directly regulates replication fork progression (Munden et al, 2018).…”

Histone concentration regulates the cell cycle and transcription in early development

“…In Drosophila, transcription factors bind their consensus sequences (Harrison et al, 2010(Harrison et al, , 2011Liang et al, 2008;Driever and Nüsslein-Volhard, 1989), RNA polymerase is gradually recruited to sites of transcription (Gaertner et al, 2012;Chen et al, 2013;Blythe and Wieschaus, 2015b) and nucleosomefree regions open on promoters (Li et al, 2014;Blythe and Wieschaus, 2016). At a larger scale, the genome becomes folded into topologically associated domains, and heterochromatin is differentiated from euchromatin for the first time (Hug et al, 2017;Shermoen et al, 2010;Seller and O'Farrell, 2018).…”

The Emerging Roles of Fox Family Transcription Factors in Chromosome Replication, Organization, and Genome Stability

“…Rif1 colocalizes specifically with the mid-S replication foci and establishes the mid-S replication domains that are restrained from being activated at early-S-phase. Rif1 prolongs the embryonic S phase at the Drosophila mid-blastula transition [45]. The Donaldson group showed that deletion of RIF1 increases the proportion of hyperphosphorylated Mcm4 and partially compensates for the limited DDK activity in a temperature-sensitive (ts) mutant of the catalytic subunit of DDK, cdc7-1, in yeast [46].…”