Rubrivivax gelatinosus acsF (Previously orf358 ) Codes for a Conserved, Putative Binuclear-Iron-Cluster-Containing Protein Involved in Aerobic Oxidative Cyclization of Mg-Protoporphyrin IX Monomethylester

Abstract

ABSTRACT This study describes the characterization of orf358 , an open reading frame of previously unidentified function, in the purple bacterium Rubrivivax gelatinosus . A strain in which orf358 was disrupted exhibited a phenotype similar to the wild type under photosynthesis or low-aeration respiratory growth conditions. In contrast, under highly aerated respiratory growth conditions, the wild type still produced bacteriochlorophyll a (Bchl a ), while the disrupted strain accumulated a compound that had the same absorption and fluorescence emission spectra as Mg-protoporphyrin but was less polar, suggesting that it was Mg-protoporphyrin monomethylester (MgPMe). These data indicated a blockage in Bchl a synthesis at the oxidative cyclization stage and implied the coexistence of two different mechanisms for MgPMe cyclization in R. gelatinosus , an anaerobic mechanism active under photosynthesis or low oxygenation and an aerobic mechanism active under high-oxygenation growth conditions. Based on these results as well as on sequence analysis indicating the presence of conserved putative binuclear-iron-cluster binding motifs, the designation of orf358 as acsF (for aerobic cyclization system Fe-containing subunit) is proposed. Several homologs of AcsF were found in a wide range of photosynthetic organisms, including Chlamydonomas reinhardtii Crd1 and Pharbitis nil PNZIP, suggesting that this aerobic oxidative cyclization mechanism is conserved from bacteria to plants.