Abstract
Commitment to cell division at the end of G1 phase, termed Start in the budding yeast Saccharomyces cerevisiae, is strongly influenced by nutrient availability. To identify new dominant activators of Start that might operate under different nutrient conditions, we screened a genome-wide ORF overexpression library for genes that bypass a Start arrest caused by absence of the G1 cyclin Cln3 and the transcriptional activator Bck2. We recovered a hypothetical gene YLR053c, renamed NSR1 for Nitrogen-responsive Start Regulator 1, which encodes a poorly characterized 108 amino acid microprotein. Endogenous Nsr1 was nuclear-localized, restricted to poor nitrogen conditions, induced upon mTORC1 inhibition, and cell cycle-regulated with a peak at Start. NSR1 interacted genetically with SWI4 and SWI6, which encode the master G1/S transcription factor complex SBF. Correspondingly, Nsr1 physically interacted with Swi4 and Swi6 and was localized to G1/S promoter DNA. Nsr1 exhibited inherent transactivation activity and fusion of Nsr1 to the SBF inhibitor Whi5 was sufficient to suppress other Start defects. Nsr1 appears to be a recently evolved microprotein that rewires the G1/S transcriptional machinery under poor nutrient conditions.
Author Summary Unicellular microorganisms must adapt to ever-changing nutrient conditions and hence must adjust cell growth and proliferation to maximize fitness. In the budding yeast Saccharomyces cerevisiae, commitment to cell division, termed Start, is heavily influenced by nutrient availability. The mechanisms of Start activation under conditions of nutrient limitation are less well characterized than under nutrient excess. To identify potential new Start regulators specific to poor nutrient environments, we screened for genes able to bypass a genetic Start arrest caused by loss of the G1 cyclin Cln3 and the transcriptional activator Bck2. This screen uncovered YLR053c, which we renamed NSR1 for Nitrogen-responsive Start Regulator. Sequence analysis across yeast species indicated that Nsr1 is a recently-evolved microprotein. We showed that NSR1 is nutrient- and cell cycle-regulated, and directly binds the main G1/S transcription factor complex SBF. We demonstrated that Nsr1 has an intrinsic trans-activation activity and provided genetic evidence to suggest that Nsr1 can bypass the requirement for normal Cln3-dependent activation of SBF. These results uncover a new mechanism of Start activation and demonstrate how microproteins can rapidly emerge to rewire fundamental cellular processes.
Competing Interest Statement
The authors have declared no competing interest.
