Decreasing Serine Levels During Growth Transition Triggers Biofilm Formation in Bacillus subtilis

Abstract

Biofilm development in Bacillus subtilis is regulated at multiple levels. While a number of known signals that trigger biofilm formation do so through the activation of one or more sensory histidine kinases, it was recently discovered that biofilm activation is also coordinated by sensing intracellular metabolic signals, including serine starvation. Serine starvation causes ribosomes to pause on specific serine codons, leading to a decrease in the translation rate of sinR, which encodes a master repressor for biofilm matrix genes, and ultimately biofilm induction. How serine levels change in different growth stages, how B. subtilis regulates intracellular serine levels in response to metabolic status, and how serine starvation triggers ribosomes to pause on selective serine codons remain unknown. Here we show that serine levels decrease as cells enter stationary phase and that unlike most other amino acid biosynthesis genes, expression of serine biosynthesis genes decreases upon the transition into stationary phase. Deletion of the gene for a serine deaminase responsible for converting serine to pyruvate led to a delay in biofilm formation, further supporting the idea that serine levels are a critical intracellular signal for biofilm activation. Finally, we show that levels of all five serine tRNA isoacceptors are decreased in stationary phase compared to exponential phase. Interestingly, the three isoacceptors recognizing UCN serine codons are reduced to a much greater extent than the two that recognize AGC and AGU serine codons. Our findings provide evidence for a link between serine homeostasis and biofilm development in B. subtilis. IMPORTANCEIn Bacillus subtilis, biofilm formation is triggered in response to various environmental and cellular signals. It was previously proposed that serine limitation acts as a proxy for nutrient status and triggers biofilm formation at the onset of biofilm entry through a novel signaling mechanism caused by global ribosome pausing on selective serine codons. In this study, we revealed that serine levels decrease at the biofilm entry due to catabolite control and a shunt mechanism. We also show that levels of five serine tRNA isoacceptors are differentially decreased in stationary phase compared to exponential phase; three isoacceptors recognizing UCN serine codons are reduced much greater than the two recognizing AGC and AGU codons. This indicates a possible mechanism for selective ribosome pausing.