In eukaryotes, in response to replication stress, DNA damage response kinase, ATR is activated, whose signalling abrogationleads to cell lethality due to aberrant fork remodelling and excessive origin firing. Here we report that inhibition ofATR kinase activity specifically during replication stress recovery results in persistent ATR signalling, evidenced by thepresence of ATR-dependent phosphorylation marks (cH2AX, pChk1 and pRad17) and delayed cell cycle re-entry. Further,such disruption of ATR signalling attenuation leads to double-strand breaks, fork collapse and thereby ‘replicationcatastrophe’. PPM1D phosphatase, a nucleolar localized protein, relocates to chromatin during replication stress and revertsback to nucleolus following stress recovery, under the control of ATR kinase action. Inhibition of ATR kinase activity,specifically during post replication stress, triggers dislodging of the chromatin-bound PPM1D from nucleus to cytoplasmfollowed by its degradation, thereby leading to persistence of activated ATR marks in the nuclei. Chemical inhibition ofPPM1D activity or SiRNA mediated depletion of the protein during post replication stress recovery ‘phenocopies’ ATRkinase inhibition by failing to attenuate ATR signalling. Collectively, our observations suggest a novel role of ATR kinasein mediating its own signal attenuation via PPM1D recruitment to chromatin as an essential mechanism for restarting thestalled forks, cell-cycle re-entry and cellular recovery from replication stress.

The rising global population is forcing the need for adapting alternative sustainable technologies for enhancedcrop productivity. The CO2 Concentration Mechanisms (CCMs) evolved in algae to counter the inefficient CO2fixing enzyme, RuBisCo and slower diffusion of CO2 in water offers good scope for the above purpose. TheCCMs are single-celled CO2 supply mechanisms that depend on multiple CO2/HCO3

- transporters andacclimation states and accumulate 100-fold more CO2 than low CO2 environments. Although some insightshave been obtained regarding the CCMs of blue-green algae and green algae like Chlamydomonas reinhardtii,further progress needs to take place to understand the molecular and biochemical basis for intracellulartransport of CO2. In this review, complete information pertaining to the core CCM is presented and discussedin light of the available literature. In addition to this, information on CO2/HCO3

- sensing, photo-acclimation inlow CO2, liquid-like nature of pyrenoid, untapped potential of high CO2 responses and high CO2 requiringmutants, and prospects of engineering CCM components into higher plants are presented and discussed.