C-type cytochromes (cyt c) are proteins that undergo post-translational modification to covalently bind heme, which allows them to facilitate redox reactions in electron transport chains across all domains of life. Genomic evidence suggests that cyt c are involved in electron transfer processes among the Archaea, especially in members that produce or consume the potent greenhouse gas methane. However, neither the maturation machinery for cyt c in Archaea nor their role in methane metabolism has ever been functionally characterized. Here, we have used CRISPR-Cas9 genome editing tools to map a distinct pathway for cyt c biogenesis in the model methanogenic archaeon Methanosarcina acetivorans, and have also identified substrate-specific functional roles for cyt c during methanogenesis. Although the cyt c maturation machinery from M. acetivorans is universally conserved in the Archaea, our evolutionary analyses indicate that different clades of Archaea acquired this machinery through multiple independent horizontal gene transfer events from different groups of Bacteria. Overall, we demonstrate the convergent evolution of a novel Archaea-specific cyt c maturation machinery and its physiological role during methanogenesis, a process which contributes substantially to global methane emissions

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Plant pathogenic Ralstonia spp. colonize plant xylem and cause wilt diseases on a broad range of host plants. To identify genes that promote growth of diverse Ralstonia strains in xylem sap from tomato plants, we performed genome-scale genetic screens (random barcoded transposon mutant sequencing screens; RB-TnSeq) in Ralstonia pseudosolanacearum GMI1000 and R. syzygii PSI07. Contrasting mutant fitness phenotypes in culture media versus in xylem sap suggest that Ralstonia strains are adapted to sap and that culture media impose foreign selective pressures. Although wild-type Ralstonia grew in sap and in rich medium with similar doubling times and to a similar carrying capacity, more genes were essential for growth in sap than in rich medium. Multiple mutants lacking amino acid biosynthesis and central metabolism functions had fitness defects in xylem sap and minimal medium. Our screen identified > 26 genes in each strain that contributed to growth in xylem sap but were dispensable for growth in culture media. Many sap-specific fitness factors are associated with bacterial stress responses: envelope remodeling and repair processes such as peptidoglycan peptide formation (murI and RSc1177), LPS O-antigen biosynthesis (RSc0684), periplasmic protein folding (dsbA), drug efflux (tolA and tolR), and stress responses (cspD3). Our genome-scale genetic screen identified Ralstonia fitness factors that promote growth in xylem sap, an ecologically relevant condition.ImportanceTraditional transposon mutagenesis genetic screens pioneered molecular plant pathology and identified core virulence traits like the type III secretion system. TnSeq approaches that leverage next-generation sequencing to rapidly quantify transposon mutant phenotypes are ushering in a new wave of biological discovery. Here we have adapted a genome-scale approach, random barcoded transposon mutant sequencing (RB-TnSeq), to discover fitness factors that promote growth of two related bacterial strains in a common niche, tomato xylem sap. Fitness of wild-type and mutants show that Ralstonia spp. are adapted to grow well in xylem sap from their natural host plant, tomato. Our screen identified multiple sap-specific fitness factors with roles in maintaining the bacterial envelope. These factors are putative adaptations to resist plant defenses, including antimicrobial proteins and specialized metabolites that damage bacterial membranes.

Traditional transposon mutagenesis genetic screens pioneered molecular plant pathology and identified core virulence traits like the type III secretion system. TnSeq approaches that leverage next-generation sequencing to rapidly quantify transposon mutant phenotypes are ushering in a new wave of biological discovery.