Journal of Bacteriology, June 1999, p. 3486-3493, Vol. 181, No. 11
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Department of Medical Microbiology,
Received 9 December 1998/Accepted 7 April 1999
The Mycobacterium tuberculosis hmp gene encodes a
protein which is homologous to flavohemoglobin in Escherichia
coli. Northern blotting analysis demonstrated that
hmp transcription increased when a microaerophilic culture
became oxygen limited as it entered stationary phase at 20 days. There
was a fivefold increase of the hmp transcripts during early
stationary phase compared with the value which was observed in the
exponential growth phase. This induction of hmp
transcription was not due to changes in the mRNA stability since the
half-life of hmp mRNA was very short in a 20-day
microaerophilic culture. No induction of hmp mRNA was
observed during entry into stationary phase when the culture was
continuously aerated. hmp transcription was induced after a
short exposure of a late-exponential-phase culture to anaerobic conditions. These data indicate that oxygen limitation is the trigger
for hmp gene transcription. In addition, when a
microaerophilic culture entered into the stationary phase at 20 days,
transcription of hmp increased to a small extent after
exposure to S-nitrosoglutathione (a nitric oxide [NO]
releaser) and sodium nitroprusside (an NO+ donor) and
decreased after exposure to paraquat (a superoxide generator) and
H2O2. In log phase (4 days) and late
stationary phase (40 days), the transcription of hmp
was unaffected by nitrosative and oxidative stress. Three primer
extension products were observed. The
10 region is 100%
identical to that of promoter T3 in mycobacteria and shows a strong
similarity to the
10 sequence of hmp and rpoS promoters in E. coli. These observations of hmp
mRNA induction in response to O2 limitation and nitrosative
stress suggest that the hmp gene of M. tuberculosis may have a role in protection of the organism from
NO killing under microaerophilic conditions.
*
Corresponding author. Mailing address: Department of
Medical Microbiology, St. George's Hospital Medical School, London
SW17 0RE, United Kingdom. Phone: 44-181-725-5725. Fax: 44-181-672-0234. E-mail: acoates{at}sghms.ac.uk.
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