His-Asp phosphorelay (also known as two-component signal transduction) proteins are the predominant mechanism used in most bacteria to control behavior in response to changing environmental conditions. In addition to systems consisting of a simple two-component system utilizing an isolated histidine kinase/response regulator pair, some bacteria are enriched in histidine kinases that serve as signal integration proteins; these kinases are usually characterized by noncanonical domain architecture, and the responses that they regulate may be difficult to identify. The environmental bacterium Myxococcus xanthus is highly enriched in these noncanonical histidine kinases. M. xanthus is renowned for a starvation-induced multicellular developmental program in which some cells are induced to aggregate into fruiting bodies and then differentiate into environmentally resistant spores. Here, we characterize the M. xanthus orphan hybrid histidine kinase SinK (Mxan_4465), which consists of a histidine kinase transmitter followed by two receiver domains (REC1 and REC2). Nonphosphorylatable sinK mutants were analyzed under two distinct developmental conditions and using a new high-resolution developmental assay. These assays revealed that SinK autophosphorylation and REC1 impact the onset of aggregation and/or the mobility of aggregates, while REC2 impacts sporulation efficiency. SinK activity is controlled by a genus-specific hypothetical protein (SinM; Mxan_4466). We propose that SinK serves to fine-tune fruiting body morphology in response to environmental conditions. IMPORTANCE Biofilms are multicellular communities of microorganisms that play important roles in host disease or environmental biofouling. Design of preventative strategies to block biofilms depends on understanding the molecular mechanisms used by microorganisms to build them. The production of biofilms in bacteria often involves two-component signal transduction systems in which one protein component (a kinase) detects an environmental signal and, through phosphotransfer, activates a second protein component (a response regulator) to change the transcription of genes necessary to produce a biofilm. We show that an atypical kinase, SinK, modulates several distinct stages of specialized biofilm produced by the environmental bacterium Myxococcus xanthus. SinK likely integrates multiple signals to fine-tune biofilm formation in response to distinct environmental conditions.

Environmental microorganisms have evolved a variety of strategies to survive fluctuations in environmental conditions, including production of biofilms and differentiation into spores.Myxococcus xanthusare ubiquitous soil bacteria that produce starvation-induced multicellular fruiting bodies filled with environmentally resistant spores (a specialized biofilm). Fruiting bodies are thought to facilitate theM. xanthussocial life cycle by ensuring spores can germinateen masseinto a productive feeding community. Isolated spores have been shown to be more resistant than vegetative cells to heat, ultraviolet radiation, and desiccation, but it is unknown whether assembly of spores into a fruiting body provides additional protection from environmental insults. We developed a high-throughput method to compare the recovery (outgrowth) of distinct cell types (vegetative cells, free spores, and intact fruiting bodies) after exposure to ultraviolet radiation or desiccation. Our data indicate haystack-shaped fruiting bodies protect spores from extended UV radiation but do not provide additional protection from desiccation. Perturbation of fruiting body morphology strongly impedes recovery from both UV exposure and desiccation. These results hint that the distinctive fruiting bodies produced by different myxobacterial species may have evolved to optimize their persistence in distinct ecological niches.

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