Novel FixL homologues in Chlamydomonas reinhardtii bind heme and O2

► Analysis of the Chlamydomonas genome identified nine FixL-like proteins. ► Recombinant FXL1 and FXL5 domains both stained positively on gels for heme. ► FXL1-H200S and FXL5-H200S resulted in the loss of heme binding. ► FXL1 and FXL5 both bind O2 with moderate affinity. ► These proteins may be similar to O2-sensing proteins of nitrogen-fixing bacteria.