Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, 430072, Hubei Province, China.
University of Chinese Academy of Sciences, Beijing, 100049, China.
BMC Microbiol. 2019 Jul 30;19(1):173. doi: 10.1186/s12866-019-1549-9.
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.
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.
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.
大多数希瓦氏菌物种拥有两个亚铁螯合酶的旁系同源物，用于合成 c 型细胞色素，这对于它们的呼吸多样性至关重要。在我们之前对希瓦氏菌 loihica PV-4 菌株的研究中，我们发现hemH1 的破坏而不是 hemH2 的破坏导致细胞外原卟啉 IX（PPIX）的大量积累，但在 Shewanella oneidensis MR-1 中则不同。因此，我们研究了 S. oneidensis MR-1 中的两个亚铁螯合酶基因 hemH1 和 hemH2 的功能和转录调控。
在本研究中，在 S. oneidensis MR-1 中缺失 hemH1 或 hemH2 并不像之前在 S. loihica PV-4 的 hemH1 突变体中那样导致细胞外原卟啉 IX（PPIX）的过度产生。此外，外源血红素的补充使得在 MR-1 中而不是在 PV-4 中生成 hemH1 和 hemH2 双突变体成为可能。在有氧条件下，外源血红素是 MR-1ΔhemH1ΔhemH2 生长所必需的，该菌也会过度产生细胞外 PPIX。这些结果表明血红素对希瓦氏菌属的需氧生长是必不可少的，MR-1 也可以摄取血红素用于合成必需的细胞色素和呼吸。此外，外源性血红素介导 CymA 细胞色素成熟和细胞内 KatB 过氧化氢酶活性。hemH 两个旁系同源物在野生型 MR-1 中都有转录，hemH2 的转录在 MR-1ΔhemH1 突变体中显著上调，以弥补 hemH1 的缺失。位于同一操纵子中的周质谷胱甘肽过氧化物酶基因 pgpD 和编码铁、血红素（血红素）摄取系统的大型基因簇在 PV-4 基因组中缺失。
我们的结果表明，这些希瓦氏菌物种之间的基因内容和基因表达的遗传分化，导致了这里描述的表型差异，可能是由于它们的物种形成和适应它们进化的特定栖息地（富含铁的深海喷口与缺铁的淡水）造成的，所产生的突变体可能被用于商业生产 PPIX。
