Dynamics of immediate adaptation to energy stress in yeast

To survive, all organisms face the challenge of providing a constant energy supply to the processes essential for life in an inconsistent environment. Saccharomyces cerevisiae shows a remarkably efficient, yet versatile, metabolism and adapts readily to nutrient availability. Glucose serves as the preferential energy source but, in response to scarcity, “diauxic shift” remodels metabolism to maintain optimum growth rates on available sources of carbon and energy. This entails a comprehensive transcriptional re-programming, in which hundreds of genes required for alternative carbon metabolism and stress-protection are induced, while expression of growth-related factors is suppressed. However, at the time of nutrient shift the cellular mRNA pool is composed of pre-existing transcripts, produced under glucose-replete conditions. As such, re-programming is complemented by rapid changes to inactivate pre-stress mRNAs while a selective transcriptional response is mounted. Global mRNA translation is dramatically reduced to minimal levels within minutes. Specific stress-induced transcripts can bypass this repression. This conserves energy, focusses key resources and prevents production of non-adaptive proteins. In this work, we discovered that this immediate protective response to energy stress is driven by a specialized metabolic shift, distinct from the known signalling networks mediating conventional glucose repression. We report that following glucose withdrawal, intracellular NTP levels drop drastically within 30 seconds, before stabilizing at a regulated set-point. Programmed translational control results from the differential NTP affinities of key enzymes; ATP falls below the high binding constants for DEAD-box helicase initiation factors, including eIF4A. This drives mRNA release, blocking further 80S ribosome assembly. Contrastingly, GTP levels remain sufficient for elongation factor activity due to their low binding constants. This allows run-off of initiated ribosomes and achieves an orderly translation shutdown. The response is triggered by energy depletion, rather than being glucose-specific, allowing inhibition of energy-consuming processes on very rapid timescales. Investigating temporal dynamics, we find a biphasic translational response allows metabolic remodelling. Initiation is immediately lost on all mRNAs following stress, driven by NTP depletion. Our results suggest this initiates transcriptional change, ensuring timely induction of adaptive mRNAs. Translation is then selectively re-established, preferentially on upregulated transcripts. We propose the small population of post-stress synthesised mRNA can be distinguished from existing bulk mRNA populations, privileging their translation. Thus, temporal co-ordination of translational and transcriptional responses by specialised metabolic signalling allows immediate adaptive responses to energy stress in yeast.