Novel FixL homologues in Chlamydomonas reinhardtii bind heme and O ₂

Genome inspection revealed nine putative heme-binding, FixL-homologous proteins in Chlamydomonas reinhardtii. The heme-binding domains from two of these proteins, FXL1 and FXL5 were cloned, expressed in Escherichia coli, purified and characterized. The recombinant FXL1 and FXL5 domains stained positively for heme, while mutations in the putative ligand-binding histidine FXL1-H200S and FXL5-H200S resulted in loss of heme binding. The FXL1 and FXL5 [Fe(II), bound O₂] had Soret absorption maxima around 415 nm, and weaker absorptions at longer wavelengths, in concurrence with the literature. Ligand-binding measurements showed that FXL1 and FXL5 bind O₂ with moderate affinity, 135 and 222 μM, respectively. This suggests that Chlamydomonas may use the FXL proteins in O₂-sensing mechanisms analogous to that reported in nitrogen-fixing bacteria to regulate gene expression.