Differential gene content and gene expression for bacterial evolution and speciation of Shewanella in terms of biosynthesis of heme and heme-requiring proteins.

Background: Most species of Shewanella harbor two ferrochelatase paralogues for the biosynthesis of c-type cytochromes, which are crucial for their respiratory versatility. In our previous study of the Shewanella loihica PV-4 strain, we found that the disruption of hemH1 but not hemH2 resulted in a significant accumulation of extracellular protoporphyrin IX (PPIX), but it is different in Shewanella oneidensis MR-1. Hence, the function and transcriptional regulation of two ferrochelatase genes, hemH1 and hemH2, are investigated in S. oneidensis MR-1.

Result: In the present study, deletion of either hemH1 or hemH2 in S. oneidensis MR-1 did not lead to overproduction of extracellular protoporphyrin IX (PPIX) as previously described in the hemH1 mutants of S. loihica PV-4. Moreover, supplement of exogenous hemins made it possible to generate the hemH1 and hemH2 double mutant in MR-1, but not in PV-4. Under aerobic condition, exogenous hemins were required for the growth of MR-1ΔhemH1ΔhemH2, which also overproduced extracellular PPIX. These results suggest that heme is essential for aerobic growth of Shewanella species and MR-1 could also uptake hemin for biosynthesis of essential cytochrome(s) and respiration. Besides, the exogenous hemin mediated CymA cytochrome maturation and the cellular KatB catalase activity. Both hemH paralogues were transcribed in wild-type MR-1, and the hemH2 transcription was remarkably up-regulated in MR-1ΔhemH1 mutant to compensate for the loss of hemH1. The periplasmic glutathione peroxidase gene pgpD, located in the same operon with hemH2, and a large gene cluster coding for iron, heme (hemin) uptake systems are absent in the PV-4 genome.

Conclusion: Our results indicate that the genetic divergence in gene content and gene expression between these Shewanella species, accounting for the phenotypic difference described here, might be due to their speciation and adaptation to the specific habitats (iron-rich deep-sea vent versus iron-poor freshwater) in which they evolved and the generated mutants could potentially be utilized for commercial production of PPIX.