Topoisomerase 1 activity during mitotic transcription favors the transition from mitosis to G1.

As cells enter mitosis, chromatin compacts to facilitate chromosome segregation yet remains transcribed. Transcription supercoils DNA to levels that can impede further progression of RNA polymerase II (RNAPII) unless it is removed by DNA topoisomerase 1 (TOP1). Using ChIP-seq on mitotic cells, we found that TOP1 is required for RNAPII translocation along genes. The stimulation of TOP1 activity by RNAPII during elongation allowed RNAPII clearance from genes in prometaphase and enabled chromosomal segregation. Disruption of the TOP1-RNAPII interaction impaired RNAPII spiking at promoters and triggered defects in the post-mitotic transcription program. This program includes factors necessary for cell growth, and cells with impaired TOP1-RNAPII interaction are more sensitive to inhibitors of mTOR signaling. We conclude that TOP1 is necessary for assisting transcription during mitosis with consequences for growth and gene expression long after mitosis is completed. In this sense, TOP1 ensures that cellular memory is preserved in subsequent generations.

Synechocystis KaiC3 Displays Temperature- and KaiB-Dependent ATPase Activity and Is Important for Growth in Darkness.

Cyanobacteria form a heterogeneous bacterial group with diverse lifestyles, acclimation strategies, and differences in the presence of circadian clock proteins. In Synechococcus elongatus PCC 7942, a unique posttranslational KaiABC oscillator drives circadian rhythms. ATPase activity of KaiC correlates with the period of the clock and mediates temperature compensation. Synechocystis sp. strain PCC 6803 expresses additional Kai proteins, of which KaiB3 and KaiC3 proteins were suggested to fine-tune the standard KaiAB1C1 oscillator. In the present study, we therefore characterized the enzymatic activity of KaiC3 as a representative of nonstandard KaiC homologs in vitro KaiC3 displayed ATPase activity lower than that of the Synechococcus elongatus PCC 7942 KaiC protein. ATP hydrolysis was temperature dependent. Hence, KaiC3 is missing a defining feature of the model cyanobacterial circadian oscillator. Yeast two-hybrid analysis showed that KaiC3 interacts with KaiB3, KaiC1, and KaiB1. Further, KaiB3 and KaiB1 reduced in vitro ATP hydrolysis by KaiC3. Spot assays showed that chemoheterotrophic growth in constant darkness is completely abolished after deletion of ΔkaiAB1C1 and reduced in the absence of kaiC3 We therefore suggest a role for adaptation to darkness for KaiC3 as well as a cross talk between the KaiC1- and KaiC3-based systems.IMPORTANCE The circadian clock influences the cyanobacterial metabolism, and deeper understanding of its regulation will be important for metabolic optimizations in the context of industrial applications. Due to the heterogeneity of cyanobacteria, characterization of clock systems in organisms apart from the circadian model Synechococcus elongatus PCC 7942 is required. Synechocystis sp. strain PCC 6803 represents a major cyanobacterial model organism and harbors phylogenetically diverged homologs of the clock proteins, which are present in various other noncyanobacterial prokaryotes. By our in vitro studies we unravel the interplay of the multiple Synechocystis Kai proteins and characterize enzymatic activities of the nonstandard clock homolog KaiC3. We show that the deletion of kaiC3 affects growth in constant darkness, suggesting its involvement in the regulation of nonphotosynthetic metabolic pathways.

Minimal tool set for a prokaryotic circadian clock.

BACKGROUND: Circadian clocks are found in organisms of almost all domains including photosynthetic Cyanobacteria, whereby large diversity exists within the protein components involved. In the model cyanobacterium Synechococcus elongatus PCC 7942 circadian rhythms are driven by a unique KaiABC protein clock, which is embedded in a network of input and output factors. Homologous proteins to the KaiABC clock have been observed in Bacteria and Archaea, where evidence for circadian behavior in these domains is accumulating. However, interaction and function of non-cyanobacterial Kai-proteins as well as homologous input and output components remain mainly unclear. RESULTS: Using a universal BLAST analyses, we identified putative KaiC-based timing systems in organisms outside as well as variations within Cyanobacteria. A systematic analyses of publicly available microarray data elucidated interesting variations in circadian gene expression between different cyanobacterial strains, which might be correlated to the diversity of genome encoded clock components. Based on statistical analyses of co-occurrences of the clock components homologous to Synechococcus elongatus PCC 7942, we propose putative networks of reduced and fully functional clock systems. Further, we studied KaiC sequence conservation to determine functionally important regions of diverged KaiC homologs. Biochemical characterization of exemplary cyanobacterial KaiC proteins as well as homologs from two thermophilic Archaea demonstrated that kinase activity is always present. However, a KaiA-mediated phosphorylation is only detectable in KaiC1 orthologs. CONCLUSION: Our analysis of 11,264 genomes clearly demonstrates that components of the Synechococcus elongatus PCC 7942 circadian clock are present in Bacteria and Archaea. However, all components are less abundant in other organisms than Cyanobacteria and KaiA, Pex, LdpA, and CdpA are only present in the latter. Thus, only reduced KaiBC-based or even simpler, solely KaiC-based timing systems might exist outside of the cyanobacterial phylum, which might be capable of driving diurnal oscillations.

Insight into cyanobacterial circadian timing from structural details of the KaiB-KaiC interaction.

Circadian timing in cyanobacteria is determined by the Kai system consisting of KaiA, KaiB, and KaiC. Interactions between Kai proteins change the phosphorylation status of KaiC, defining the phase of circadian timing. The KaiC-KaiB interaction is crucial for the circadian rhythm to enter the dephosphorylation phase but it is not well understood. Using mass spectrometry to characterize Kai complexes, we found that KaiB forms monomers, dimers, and tetramers. The monomer is the unit that interacts with KaiC, with six KaiB monomers binding to one KaiC hexamer. Hydrogen-deuterium exchange MS reveals structural changes in KaiC upon binding of KaiB in both the CI and CII domains, showing allosteric coupling upon KaiB binding. Based on this information we propose a model of the KaiB-KaiC complex and hypothesize that the allosteric changes observed upon complex formation relate to coupling KaiC ATPase activity with KaiB binding and to sequestration of KaiA dimers into KaiCBA complexes.

Daily expression pattern of protein-encoding genes and small noncoding RNAs in synechocystis sp. strain PCC 6803.

Many organisms harbor circadian clocks with periods close to 24 h. These cellular clocks allow organisms to anticipate the environmental cycles of day and night by synchronizing circadian rhythms with the rising and setting of the sun. These rhythms originate from the oscillator components of circadian clocks and control global gene expression and various cellular processes. The oscillator of photosynthetic cyanobacteria is composed of three proteins, KaiA, KaiB, and KaiC, linked to a complex regulatory network. Synechocystis sp. strain PCC 6803 possesses the standard cyanobacterial kaiABC gene cluster plus multiple kaiB and kaiC gene copies and antisense RNAs for almost every kai transcript. However, there is no clear evidence of circadian rhythms in Synechocystis sp. PCC 6803 under various experimental conditions. It is also still unknown if and to what extent the multiple kai gene copies and kai antisense RNAs affect circadian timing. Moreover, a large number of small noncoding RNAs whose accumulation dynamics over time have not yet been monitored are known for Synechocystis sp. PCC 6803. Here we performed a 48-h time series transcriptome analysis of Synechocystis sp. PCC 6803, taking into account periodic light-dark phases, continuous light, and continuous darkness. We found that expression of functionally related genes occurred in different phases of day and night. Moreover, we found day-peaking and night-peaking transcripts among the small RNAs; in particular, the amounts of kai antisense RNAs correlated or anticorrelated with those of their respective kai target mRNAs, pointing toward the regulatory relevance of these antisense RNAs. Surprisingly, we observed that the amounts of 16S and 23S rRNAs in this cyanobacterium fluctuated in light-dark periods, showing maximum accumulation in the dark phase. Importantly, the amounts of all transcripts, including small noncoding RNAs, did not show any rhythm under continuous light or darkness, indicating the absence of circadian rhythms in Synechocystis.

Biochemical analysis of three putative KaiC clock proteins from Synechocystis sp. PCC 6803 suggests their functional divergence.

Cyanobacteria have been shown to have a circadian clock system that consists mainly of three protein components: KaiA, KaiB and KaiC. This system is well understood in the cyanobacterium Synechococcus elongatus PCC 7942, for which robust circadian oscillations have been shown. Like many other cyanobacteria, the chromosome of the model cyanobacterium Synechocystis sp. PCC 6803 contains additional kaiC and kaiB gene copies besides the standard kaiABC gene cluster. The respective gene products differ significantly in their amino acid sequences, especially in their C-terminal regions, suggesting different functional characteristics. Here, phosphorylation assays of the three Synechocystis sp. PCC 6803 KaiC proteins revealed that KaiC1 phosphorylation depends on KaiA, as is well documented for the Synechococcus elongatus PCC 7942 KaiC protein, whereas KaiC2 and KaiC3 autophosphorylate independently of KaiA. This was confirmed by in vivo protein-protein interaction studies, which demonstrate that only KaiC1 interacts with KaiA. Furthermore, we demonstrate that the three different Kai proteins form only homomeric complexes in vivo. As only KaiC1 phosphorylation depends on KaiA, a prerequisite for robust oscillations, we suggest that the kaiAB1C1 gene cluster in Synechocystis sp. PCC 6803 controls circadian timing in a manner similar to the clock described in Synechococcus elongatus PCC 7942.

Coinhibition of topoisomerase 1 and BRD4-mediated pause release selectively kills pancreatic cancer via readthrough transcription.

Pancreatic carcinoma lacks effective therapeutic strategies resulting in poor prognosis. Transcriptional dysregulation due to alterations in KRAS and MYC affects initiation, development, and survival of this tumor type. Using patient-derived xenografts of KRAS- and MYC-driven pancreatic carcinoma, we show that coinhibition of topoisomerase 1 (TOP1) and bromodomain-containing protein 4 (BRD4) synergistically induces tumor regression by targeting promoter pause release. Comparing the nascent transcriptome with the recruitment of elongation and termination factors, we found that coinhibition of TOP1 and BRD4 disrupts recruitment of transcription termination factors. Thus, RNA polymerases transcribe downstream of genes for hundreds of kilobases leading to readthrough transcription. This occurs during replication, perturbing replisome progression and inducing DNA damage. The synergistic effect of TOP1 + BRD4 inhibition is specific to cancer cells leaving normal cells unaffected, highlighting the tumor's vulnerability to transcriptional defects. This preclinical study provides a mechanistic understanding of the benefit of combining TOP1 and BRD4 inhibitors to treat pancreatic carcinomas addicted to oncogenic drivers of transcription and replication.

TOP1 CAD-seq: A protocol to map catalytically engaged topoisomerase 1 in human cells.

TOP1 CAD-seq enables mapping of TOP1 sites of covalent engagement with DNA. The procedure depends upon enrichment of DNA-covalent adducts using chaotropic salts and immunoprecipitation with an antibody specific for TOP1. Here, we describe a step-by-step protocol compatible with Illumina sequencing and bioinformatic pipeline for preliminary data analysis. Compared to other approaches for the genomic study of topoisomerases, TOP1 CAD-seq provides information about active TOP1 engaged on the DNA, taking advantage of low background due to absence of crosslinking. For complete details on the use and execution of this protocol, please refer to Das et al. (2022).

Structures of the cyanobacterial circadian oscillator frozen in a fully assembled state.

Cyanobacteria have a robust circadian oscillator, known as the Kai system. Reconstituted from the purified protein components KaiC, KaiB, and KaiA, it can tick autonomously in the presence of adenosine 5'-triphosphate (ATP). The KaiC hexamers enter a natural 24-hour reaction cycle of autophosphorylation and assembly with KaiB and KaiA in numerous diverse forms. We describe the preparation of stoichiometrically well-defined assemblies of KaiCB and KaiCBA, as monitored by native mass spectrometry, allowing for a structural characterization by single-particle cryo-electron microscopy and mass spectrometry. Our data reveal details of the interactions between the Kai proteins and provide a structural basis to understand periodic assembly of the protein oscillator.

Diversity of KaiC-based timing systems in marine Cyanobacteria.

The coordination of biological activities into daily cycles provides an important advantage for the fitness of diverse organisms. Most eukaryotes possess an internal clock ticking with a periodicity of about one day to anticipate sunrise and sunset. The 24-hour period of the free-running rhythm is highly robust against many changes in the natural environment. Among prokaryotes, only Cyanobacteria are known to harbor such a circadian clock. Its core oscillator consists of just three proteins, KaiA, KaiB, and KaiC that produce 24-hour oscillations of KaiC phosphorylation, even in vitro. This unique three-protein oscillator is well documented for the freshwater cyanobacterium Synechococcus elongatus PCC 7942. Several physiological studies demonstrate a circadian clock also for other Cyanobacteria including marine species. Genes for the core clock components are present in nearly all marine cyanobacterial species, though there are large differences in the specific composition of these genes. In the first section of this review we summarize data on the model circadian clock from S. elongatus PCC 7942 and compare it to the reduced clock system of the marine cyanobacterium Prochlorococcus marinus MED4. In the second part we discuss the diversity of timing mechanisms in other marine Cyanobacteria with regard to the presence or absence of different components of the clock.