Prodiginine Production inStreptomyces coelicolorCorrelates Temporally and Spatially to Programmed Cell Death

Programmed cell death (PCD) is a common feature of multicellularity and morphogenesis in bacteria. While cell death has been well documented when Streptomyces species switch from vegetative (nutrition) to aerial (reproduction) growth, lethal determinants are yet to be discovered to unveil the genetic basis of PCD in mycelial bacteria. In this work we used prodiginines of Streptomyces coelicolor as model to test the hypothesis that a bacterium uses ‘self-made’ antiproliferative DNA-damaging agents as toxins of their PCD process. Spatio-temporal visualisation of the autofluorescence of prodiginines reveals that their biosynthesis is triggered in the dying zone of the colony prior to morphological differentiation of the mycelium. A prodiginine nonproducer showed hyper-accumulation of viable filaments, with increased RNA and proteins synthesis when most of the mycelium of the wild-type strain was dead when prodiginine accumulated. Addition of a prodiginine synthesis inhibitor also strongly favoured viable over dead filaments. As self-toxicity has also been reported for other producers of DNA-damaging agents we propose that cytotoxic metabolites synthetized during the morphological transition of filamentous bacteria may be used to execute PCD. Significance Statement Actinobacteria are prolific producers of compounds with antiproliferative activity, but why these bacteria synthetize metabolites with this bioactivity has so far remained a mystery. Using prodiginines (PdGs) as model system, we revealed that the spatio-temporal synthesis of these molecules correlates to cell death of the producer Streptomyces coelicolor and that inhibition of their synthesis results in hyper-accumulation of viable filaments. Since PdGs potentiate death of S. coelicolor recurrently prior to morphological differentiation, this is a form of programmed cell death (PCD). Hence, next to weapons in competition between organisms or signals in inter- and intra-species communications, we propose a third role for secondary metabolites i.e., elements required for self-toxicity in PCD processes.