Abstract
The alarmone (p)ppGpp is commonly used by bacteria to quickly respond to nutrient starvation. Although (p)ppGpp synthetases such as SpoT have been extensively studied, little is known about the molecular mechanisms stimulating alarmone synthesis upon starvation. Here, we describe an essential role of the nitrogen-related phosphotransferase system (PTSNtr) in controlling (p)ppGpp accumulation in Caulobacter crescentus. We show that cells sense nitrogen starvation by way of detecting glutamine deprivation using the first enzyme (EINtr) of PTSNtr. Decreasing intracellular glutamine concentration triggers phosphorylation of EINtr and its downstream components HPr and EIIANtr. Once phosphorylated, both HPr∼P and EIIANtr∼P stimulate (p)ppGpp accumulation by modulating SpoT activities. This burst of second messenger primarily impacts the non-replicative phase of the cell cycle by extending the G1 phase. This work highlights a new role for bacterial PTS systems in stimulating (p)ppGpp accumulation in response to metabolic cues and in controlling cell cycle progression and cell growth.
Highlights
The alarmone (p)ppGpp is commonly used by bacteria to quickly respond to nutrient starvation
Previous studies showed that nitrogen starvation extends the swarmer cell lifetime in C. crescentus[4,5,6]
To understand how nitrogen starvation affects the differentiation of G1/swarmer cells, we focused our work on proteins involved in nitrogen assimilation and metabolism
Summary
The alarmone (p)ppGpp is commonly used by bacteria to quickly respond to nutrient starvation. To face the environmental changes, organisms have developed complex regulatory mechanisms that integrate stimuli and stresses Once activated, these signalling pathways modulate essential cellular processes such as DNA replication, cell division or cell growth. Other bacteria take advantage of their asymmetric cell division to adapt to starvation conditions It is the case of the aquatic a-proteobacterium Caulobacter crescentus that divides asymmetrically to give birth to two different daughter cells: a chemotactically active motile swarmer cell and a sessile stalked cell. There is no NADP-dependent glutamate dehydrogenase encoded in the genome of C. crescentus, suggesting that the assimilation of inorganic nitrogen is strictly dependent on the glutamine synthetase (GlnA) activity. In nitrogen-replete ( þ N) conditions, GlnB inhibits the transcription of glnA, by stimulating dephosphorylation of the transcriptional activator NtrC, and promotes the addition of the adenine monophosphate groups by GlnE to GlnA, which slows down the glutamine synthetase activity (Fig. 1b)
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