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J. Bacteriol., 11 1995, 6422-6431, Vol 177, No. 22
JH Zeilstra-Ryalls and S Kaplan
In Rhodobacter sphaeroides 2.4.1, the cellular requirements for 5-
aminolevulinic acid (ALA) are in part regulated by the level of ALA
synthase activity, which is encoded by the hemA and hemT genes. Under
standard growth conditions, only the hemA gene is transcribed, and the
level of ALA synthase activity varies in response to oxygen tension. The
presence of an FNR consensus sequence upstream of hemA suggested that
oxygen regulation of hemA expression could be mediated, in part, through a
homolog of the fnr gene. Two independent studies, one detailed here,
identified a region of the R. sphaeroides 2.4.1 genome containing extensive
homology to the fix region of the symbiotic nitrogen-fixing bacteria
Rhizobium meliloti and Bradyrhizobium japonicum. Within this region that
maps to 443 kbp on chromsome I, we have identified an fnr homolog (fnrL),
as well as a gene that codes for an anaerobic coproporphyrinogen III
oxidase, the second such gene identified in this organism. We also present
an analysis of the role of fnrL in the physiology of R. sphaeroides 2.4.1
through the construction and characterization of fnrL-null strains. Our
results further show that fnrL is essential for both photosynthetic and
anaerobic-dark growth with dimethyl sulfoxide. Analysis of hemA expression,
with hemA::lacZ transcriptional fusions, suggests that FnrL is an activator
of hemA under anaerobic conditions. On the other hand, the open reading
frame immediately upstream of hemA appears to be an activator of hemA
transcription regardless of either the presence or the absence of oxygen or
FnrL. Given the lack of hemT expression under these conditions, we consider
FnrL regulation of hemA expression to be a major factor in bringing about
changes in the level of ALA synthase activity in response to changes in
oxygen tension.
Copyright © 1995, American Society for Microbiology
Aerobic and anaerobic regulation in Rhodobacter sphaeroides 2.4.1: the role of the fnrL gene
Department of Microbiology and Molecular Genetics, University of Texas Health Science Center at Houston 77225, USA.
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