Photomorphogenesis of Myxococcus macrosporus: new insights for light-regulation of cell development

Myxobacteria are non-photosynthetic bacteria distinguished among prokaryotes by a multicellular stage in their life cycle known as fruiting bodies that are formed in response to nutrient deprivation and stimulated by light. Here, we report an entrained, rhythmic pattern of Myxococcus macrosporus fruiting bodies, forming consistently spaced concentric rings when grown in the dark. Light exposure disrupts this rhythmic phenotype, resulting in a sporadic arrangement and reduced fruiting-body count. M. macrosporus genome encodes a red-light photoreceptor, a bacteriophytochrome (BphP), previously shown to affect the fruiting-body formation in the related myxobacterium Stigmatella aurantiaca. Similarly, the formation of M. macrosporus fruiting bodies is also impacted by the exposure to BphP-specific wavelengths of light. RNA-Seq analysis of M. macrosporus revealed constitutive expression of the bphP gene. Phytochromes, as light-regulated enzymes, control many aspects of plant development including photomorphogenesis. They are intrinsically correlated to circadian clock proteins, impacting the overall light-mediated entrainment of the circadian clock. However, this functional relationship remains unexplored in non-photosynthetic prokaryotes. Genomic analysis unveiled the presence of multiple homologs of cyanobacterial core oscillatory gene, kaiC, in various myxobacteria, including M. macrosporus, S. aurantiaca and M. xanthus. RNA-Seq analysis verified the expression of all kaiC homologs in M. macrosporus and the closely related M. xanthus, which lacks bphP genes. Overall, this study unravels the rhythmic growth pattern during M. macrosporus development, governed by environmental factors such as light and nutrients. In addition, myxobacteria may have a time-measuring mechanism resembling the cyanobacterial circadian clock that links the photoreceptor (BphP) function to the observed rhythmic behavior.