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Indole-3-Acetic Acid Biosynthesis Is Deficient in Gluconacetobacter diazotrophicus Strains with Mutations in Cytochrome c Biogenesis Genes. Sunhee Lee, 2004.Gluconacetobacter diazotrophicus is an endophyte of sugarcane frequently found in plants grown in agricultural areas where nitrogen fertilizer input is low . Recent results from this laboratory, using mutant strains of G . diazotrophicus unable to fix nitrogen, suggested that there are two beneficial effects of G . diazotrophicus on sugarcane growth: one dependent and one not dependent on nitrogen fixation . A plant growth-promoting substance, such as indole-3-acetic acid (IAA), known to be produced by G . diazotrophicus, could be a nitrogen fixation-independent factor . One strain, MAd10, isolated by screening a library of Tn5 mutants, released only  6%
of the amount of IAA excreted by the parent strain in liquid culture .
The mutation causing the IAA– phenotype was not linked to
Tn5 . A pLAFR3 cosmid clone that complemented the IAA
deficiency was isolated . Sequence analysis of a complementing
subclone indicated the presence of genes involved in cytochrome c
biogenesis (ccm, for cytochrome c maturation) . The G .
diazotrophicus ccm operon was sequenced; the individual ccm
gene products were 37 to 52% identical to ccm gene products of
Escherichia coli and equivalent cyc genes of
Bradyrhizobium japonicum . Although several ccm mutant
phenotypes have been described in the literature, there are no
reports of ccm gene products being involved in IAA production .
Spectral analysis, heme-associated peroxidase activities, and
respiratory activities of the cell membranes revealed that the ccm
genes of G . diazotrophicus are involved in cytochrome c
biogenesis .
Mutations in the Thiol-Disulfide Oxidoreductases BdbC and BdbD Can Suppress Cytochrome c Deficiency of CcdA-Defective Bacillus subtilis Cells. Lý, 2002.Cytochromes of the c type in the gram-positive bacterium Bacillus subtilis are all membrane anchored, with their heme domains exposed on the outer side of the cytoplasmic membrane . They are distinguished from other cytochromes by having heme covalently attached by two thioether bonds . The cysteinyls in the heme-binding site (CXXCH) in apocytochrome c must be reduced in order for the covalent attachment of the heme to occur . It has been proposed that CcdA, a membrane protein, transfers reducing equivalents from thioredoxin in the cytoplasm to proteins on the outer side of the cytoplasmic membrane . Strains deficient in the CcdA protein are defective in cytochrome c and spore synthesis . We have discovered that mutations in the bdbC and bdbD genes can suppress the defects caused by lack of CcdA . BdbC and BdbD are thiol-disulfide oxidoreductases . Our experimental findings indicate that these B . subtilis proteins functionally correspond to the well-characterized Escherichia coli DsbB and DsbA proteins, which catalyze the formation of disulfide bonds in proteins in the periplasmic space . A Suppressor of the Menadione-Hypersensitive Phenotype of a Xanthomonas campestris pv . phaseoli oxyR Mutant Reveals a Novel Mechanism of Toxicity and the Protective Role of Alkyl Hydroperoxide Reductase. Paiboon Vattanaviboon, 2003.We isolated menadione-resistant mutants of Xanthomonas campestris pv . phaseoli oxyR (oxyRXp) . The oxyRR2Xp mutant was hyperresistant to the superoxide generators menadione and plumbagin and was moderately resistant to H2O2 and tert-butyl hydroperoxide . Analysis of enzymes involved in oxidative-stress protection in the oxyRR2Xp mutant revealed a >10-fold increase in AhpC and AhpF levels, while the levels of superoxide dismutase (SOD), catalase, and the organic hydroperoxide resistance protein (Ohr) were not significantly altered . Inactivation of ahpC in the oxyRR2Xp mutant resulted in increased sensitivity to menadione killing . Moreover, high levels of expression of cloned ahpC and ahpF in the oxyRXp mutant complemented the menadione hypersensitivity phenotype . High levels of other oxidant-scavenging enzymes such as catalase and SOD did not protect the cells from menadione toxicity . These data strongly suggest that the toxicity of superoxide generators could be mediated via organic peroxide production and that alkyl hydroperoxide reductase has an important novel function in the protection against the toxicity of these compounds in X . campestris.
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