John D. Hellman is a native Californian who currently resides in the beautiful finger lakes region of upstate New York (Ithaca). He earned bachelor's degrees in Chemistry and Biology (University of California, Santa Cruz) in 1982, and a PhD in Biochemistry in 1987 (U.C. Berkeley) with Dr. M.J. Chamberlin.  He then worked as a post-doctoral fellow with Dr. C. T. Walsh (Harvard Medical School) prior to joining the Department of Microbiology at Cornell in 1990 where he is now a full Professor. In addition to his research and teaching efforts, he serves as Editor-in-chief for Molecular Microbiology, as an Assoc. Editor for the Archives of Microbiology, and a member of the editorial board for the Journal of Bacteriology.

Research in John’s laboratory is focused on bacterial stress responses using Bacillus subtilis as a model system. Specifically, he investigates the global patterns of transcriptional control and the mechanisms of the corresponding regulatory proteins and pathways. The laboratory has a long standing interest in bacterial sigma factors and helped to define the biochemical role of sigma factor in promoter melting. In addition, he has investigated the roles of extracytoplasmic function sigma factors in cell envelope stress responses. A second area of interest is metal ion homeostasis and he has identified and characterized regulators involved in iron (Fur, FsrA, Btr), zinc (Zur, CzrA), manganese (MntR), and copper (CsoR) homeostasis. A third project is centered around defining oxidative stress responses and involves the PerR and OhrR regulators and the function of the recently identified low molecular weight thiol, bacillithiol.

Abstract

Oxidative Stress Responses in Bacillus subtilis

Bacillus subtilis exhibits complex and overlapping adaptive responses to oxidative stress reagents including peroxides and thiol-reactive electrophiles. Most peroxide-induced genes are controlled by PerR, SigB, and OhrR. PerR, the prototype for a widespread family of metal-dependent peroxide sensors, controls the adaptive response to hydrogen peroxide including catalase, alkylhydroperoxide reductase and a DNA-binding, iron-scavenging protein MrgA. OhrR represses expression of an inducible peroxiredoxin, OhrA. The SigB general stress regulon includes genes that overlap in function with those regulated by both PerR (e.g. katB, dps) and OhrR (e.g. ohrB). Analyses of the PerR and OhrR regulators have revealed novel sensing mechanisms and led to the discovery of bacillithiol (BSH), an abundant thiol produced by Bacilli. H2O2 selectively inactivates PerR by iron-catalysed protein oxidation of histidine 37 (H37) or H91, two residues that coordinate iron. Organic peroxides selectively inactivate OhrR. Oxidation of OhrR is mechanistically similar to other sensors of oxidative stress (e.g. OxyR, Spx, SarZ) that use reactive thiolates. Regulators controlled by S-thiolation or S-alkylation are particularly suited for the detection of compounds that deplete cellular thiol pools (disulfide stress) or function as reactive electrophiles.  In OhrR, the sole cysteine is oxidized first to the sulfenate, but this form retains DNA-binding activity. Subsequent protein S-thiolation mediates derepression. BSH was initially detected by its ability to form a mixed disulfide with OhrR. The determination of the structure of BSH has now allowed the identification of the biosynthetic pathway and the construction of cells lacking BSH. Analysis of BSH minus cells reveals the key role that this thiol plays in resistance to a variety of oxidants and also suggests a role in metal ion homeostasis. Phylogenomic profiling indicates that cells producing BSH also contain a distinct set of enzymes that function as either BSH-S-transferases or protein disulfide reductases.

keywords

peroxide stress, perR, ohrR, bacillithiol