Cytochrome c4 is required for siderophore expression by Legionella pneumophila, whereas cytochromes c1 and c5 promote intracellular infection

A panel of cytochrome c maturation (ccm) mutants of Legionella pneumophila displayed a loss of siderophore (legiobactin) expression, as measured by both the chrome azurol S assay and a Legionella-specific bioassay. These data, coupled with the finding that ccm transcripts are expressed by wild-type bacteria grown in deferrated medium, indicate that the Ccm system promotes siderophore expression by L. pneumophila. To determine the basis of this newfound role for Ccm, we constructed and tested a set of mutants specifically lacking individual c-type cytochromes. Whereas ubiquinol-cytochrome c reductase (petC) mutants specifically lacking cytochrome c, and cycB mutants lacking cytochrome c(5) had normal siderophore expression, cyc4 mutants defective for cytochrome c(4) completely lacked legiobactin. These data, along with the expression pattern of cyc4 mRNA, indicate that cytochrome c(4) in particular promotes siderophore expression. In intracellular infection assays, petC mutants and cycB mutants, but not cyc4 mutants, had a reduced ability to infect both amoebae and macrophage hosts. Like ccm mutants, the cycB mutants were completely unable to grow in amoebae, highlighting a major role for cytochrome c(5) in intracellular infection. To our knowledge, these data represent both the first direct documentation of the importance of a c-type cytochrome in expression of a biologically active siderophore and the first insight into the relative importance of c-type cytochromes in intracellular infection events.