Abstract
Hemoglobin homologs are being identified in an expanding number of unicellular prokaryotic and eukaryotic organisms. Many of these hemoglobins are twodomain proteins that possess a flavin-containing reductase in their C terminus. Determination of a function for these flavohemoglobins has been elusive. A Salmonella typhimurium strain harboring a deletion in the flavohemoglobin gene shows no difference in growth under oxidative stress conditions but displays an increased sensitivity to acidified nitrite and S-nitrosothiols, both of which produce nitric oxide. The effect is seen aerobically or anaerobically, indicating that oxygen is not required for flavohemoglobin function. These results suggest a role for the bacterial flavohemoglobins that is independent of oxygen metabolism and provide evidence for a bacterial route of protection from nitric oxide that is distinct from oxidative stress responses.
Highlights
The initial assessments of hemoglobins found in microorganisms have emphasized the diffusion and metabolism of oxygen
To determine if flavohemoglobins are involved in protection from oxidative and/or nitric oxide-mediated stress, we cloned and deleted the hmp gene of S. typhimurium
The flavohemoglobin gene is hypothesized to have been present in organisms before the divergence of prokaryotes and eukaryotes [36] and, like cytochrome oxidase, may predate the existence of significant levels of atmospheric oxygen [37]
Summary
Vol 273, No 20, Issue of May 15, pp. 12543–12547, 1998 Printed in U.S.A. (Received for publication, December 19, 1997, and in revised form, March 5, 1998). A Salmonella typhimurium strain harboring a deletion in the flavohemoglobin gene shows no difference in growth under oxidative stress conditions but displays an increased sensitivity to acidified nitrite and S-nitrosothiols, both of which produce nitric oxide. The effect is seen aerobically or anaerobically, indicating that oxygen is not required for flavohemoglobin function These results suggest a role for the bacterial flavohemoglobins that is independent of oxygen metabolism and provide evidence for a bacterial route of protection from nitric oxide that is distinct from oxidative stress responses. A sensitivity to certain oxidative stress conditions has been reported for a S. cerevisiae flavohemoglobin deletion strain lacking functional mitochondria [15]. The superoxide generator paraquat (methyl viologen) stimulates E. coli hmp promoter activity, but sensitivity of a flavohemoglobin deletion strain to superoxide was not reported [16]. We report a role for the Salmonella hmp in specific protection from nitric oxide that is independent of oxygen and its metabolites
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