<
title>
ABSTRACT<
/title>
<
p>
Determining the function and regulation of paralogues is important in understanding microbial functional genomics and environmental adaptation. Heme homeostasis is crucial for the survival of environmental microorganisms. Most<
named-content content-type='genus-species'>
Shewanella<
/named-content>
species encode two paralogues of ferrochelatase, the terminal enzyme in the heme biosynthesis pathway. The function and transcriptional regulation of two ferrochelatase genes,<
italic>
hemH1<
/italic>
and<
italic>
hemH2<
/italic>
, were investigated in<
named-content content-type='genus-species'>
Shewanella<
/named-content>
<
italic>
loihica<
/italic>
PV-4. The disruption of<
italic>
hemH1<
/italic>
but not<
italic>
hemH2<
/italic>
resulted in a significant accumulation of extracellular protoporphyrin IX (PPIX), the precursor to heme, and decreased intracellular heme levels.<
italic>
hemH1<
/italic>
was constitutively expressed, and the expression of<
italic>
hemH2<
/italic>
increased when<
italic>
hemH1<
/italic>
was disrupted. The transcription of<
italic>
hemH1<
/italic>
was regulated by the housekeeping sigma factor RpoD and potentially regulated by OxyR, while<
italic>
hemH2<
/italic>
appeared to be regulated by the oxidative stress-associated sigma factor RpoE2. When an oxidative stress condition was mimicked by adding H<
sub>
2<
/sub>
O<
sub>
2<
/sub>
to the medium or exposing the culture to light, PPIX accumulation was suppressed in the ?<
italic>
hemH1<
/italic>
mutant. Consistently, transcriptome analysis indicated enhanced iron uptake and suppressed heme synthesis in the ?<
italic>
hemH1<
/italic>
mutant. These data indicate that the two paralogues are functional in the heme synthesis pathway but regulated by environmental conditions, providing insights into the understanding of bacterial response to environmental stresses and a great potential to commercially produce porphyrin compounds.<
/p>
<
p>
<
bold>
IMPORTANCE<
/bold>
<
named-content content-type='genus-species'>
Shewanella<
/named-content>
is capable of utilizing a variety of electron acceptors for anaerobic respiration because of the existence of multiple<
italic>
c<
/italic>
-type cytochromes in which heme is an essential component. The cytochrome-mediated electron transfer across cellular membranes could potentially be used for biotechnological purposes, such as electricity generation in microbial fuel cells and dye decolorization. However, the mechanism underlying the regulation of biosynthesis of heme and cytochromes is poorly understood. Our study has demonstrated that two ferrochelatase genes involved in heme biosynthesis are differentially regulated in response to environmental stresses, including light and reactive oxygen species. This is an excellent example showing how bacteria have evolved to maintain cellular heme homeostasis. More interestingly, the high yields of extracellular protoporphyrin IX by the<
named-content content-type='genus-species'>
Shewanella loihica<
/named-content>
PV-4 mutants could be utilized for commercial production of this valuable chemical via bacterial fermentation.<
/p>