Abstract
In Bacillus subtilis, biofilm and sporulation pathways are both controlled by a master regulator, Spo0A, which is activated by phosphorylation via a phosphorelay-a cascade of phosphotransfer reactions commencing with autophosphorylation of histidine kinases KinA, KinB, KinC, KinD, and KinE. However, it is unclear how the kinases, despite acting via the same regulator, Spo0A, differentially regulate downstream pathways, i.e., how KinA mainly activates sporulation genes and KinC mainly activates biofilm genes. In this work, we found that KinC also downregulates sporulation genes, suggesting that KinC has a negative effect on Spo0A activity. To explain this effect, with a mathematical model of the phosphorelay, we revealed that unlike KinA, which always activates Spo0A, KinC has distinct effects on Spo0A at different growth stages: during fast growth, KinC acts as a phosphate source and activates Spo0A, whereas during slow growth, KinC becomes a phosphate sink and contributes to decreasing Spo0A activity. However, under these conditions, KinC can still increase the population-mean biofilm matrix production activity. In a population, individual cells grow at different rates, and KinC would increase the Spo0A activity in the fast-growing cells but reduce the Spo0A activity in the slow-growing cells. This mechanism reduces single-cell heterogeneity of Spo0A activity, thereby increasing the fraction of cells that activate biofilm matrix production. Thus, KinC activates biofilm formation by controlling the fraction of cells activating biofilm gene expression. IMPORTANCE In many bacterial and eukaryotic systems, multiple cell fate decisions are activated by a single master regulator. Typically, the activities of the regulators are controlled posttranslationally in response to different environmental stimuli. The mechanisms underlying the ability of these regulators to control multiple outcomes are not understood in many systems. By investigating the regulation of Bacillus subtilis master regulator Spo0A, we show that sensor kinases can use a novel mechanism to control cell fate decisions. By acting as a phosphate source or sink, kinases can interact with one another and provide accurate regulation of the phosphorylation level. Moreover, this mechanism affects the cell-to-cell heterogeneity of the transcription factor activity and eventually determines the fraction of different cell types in the population. These results demonstrate the importance of intercellular heterogeneity for understanding the effects of genetic perturbations on cell fate decisions. Such effects can be applicable to a wide range of cellular systems. Typical Friedreich ataxia (FRDA) is characterized by progressive ataxia with onset from early childhood to early adulthood with mean age at onset from 10 to 15 years (range: age two years to the eighth decade). Ataxia, manifesting initially as poor balance when walking, is typically followed by upper-limb ataxia, dysarthria, dysphagia, peripheral motor and sensory neuropathy, spasticity, autonomic disturbance, and often abnormal eye movements and optic atrophy. Hypertrophic cardiomyopathy is present in about two thirds of individuals; occasionally it is diagnosed prior to the onset of ataxia. Diabetes mellitus and impaired glucose tolerance can also occur. Among individuals with FRDA, about 75% have “typical Friedreich ataxia” and about 25% of individuals with biallelic FXN full-penetrance GAA repeat expansions have “atypical Friedreich ataxia” that includes late-onset FRDA (LOFA) (i.e., onset after age 25 years), very late-onset FRDA (VLOFA) (i.e., onset after age 40 years), and FRDA with retained reflexes (FARR). The diagnosis of Friedreich ataxia is established in a proband with suggestive findings and biallelic pathogenic variants in FXN identified by molecular genetic testing. The two classes of FXN pathogenic variants are (1) GAA repeat expansions and (2) FXN pathogenic sequence variants, including base substitutions and small indels or large deletions. Approximately 96% of individuals with FRDA have biallelic FXN GAA repeat expansions in intron 1; approximately 4% are compound heterozygotes for an FXN GAA repeat expansion and either an intragenic FXN pathogenic variant or a large deletion. Targeted therapy: Omaveloxolone, an Nrf2 activator, has been shown to slow the progression of FRDA; it is approved in the United States and Europe for individuals age 16 years and older. Supportive care: Multidisciplinary care by specialists in relevant fields, such as neurologists, ophthalmologists, orthoptists, physical therapists, occupational therapists, cardiologists, endocrinologists, speech and language therapists, and psychologists. Surveillance: Routinely scheduled evaluations by the treating multidisciplinary specialists. Agents/circumstances to avoid: Use and misuse of illegal and controlled drugs, as they may affect neuronal well-being and, thus, exacerbate disease manifestations; medications that are toxic or potentially toxic to people with neuropathy; circumstances that increase the risk of falling (e.g., rough surfaces). Evaluation of relatives at risk: If at-risk minor and adult sibs of an individual with FRDA have not had testing for the FXN pathogenic variant(s) in their family, they should be offered echocardiography surveillance to determine if treatable cardiac manifestations of presymptomatic disease are present. Pregnancy management: Worsening, improving, or unchanged manifestations during pregnancy were each reported with equal frequency by women with FRDA. Close cardiac monitoring and regular testing for diabetes mellitus during pregnancy is recommended in any woman with FRDA. If cesarean section is required, epidural or spinal anesthesia is recommended rather than general anesthesia if possible. FRDA is inherited in an autosomal recessive manner. If both parents are heterozygous for a pathogenic variant in FXN, each sib of an affected individual has at conception a 25% chance of inheriting biallelic FRDA-related genetic alterations, a 50% chance of inheriting one FRDA-related genetic alteration, and a 25% chance of inheriting neither of the familial FRDA-related genetic alterations. Sibs who inherit biallelic FXN pathogenic variants will be affected. Once the FRDA-related genetic alterations have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible.