Lead toxicity regulation via protein degradation and tetrapyrrole biosynthesis pathways in Brassica species: A comparative quantitative analysis of proteomic study.

Understanding the heavy metals (HMs) tolerance mechanism is crucial for improving plant growth in metal-contaminated soil. In order to evaluate the lead (Pb) tolerance mechanism in Brassica species, a comparative proteomic study was used. Thirteen-day-old seedlings of B. juncea and B. napus were treated with different Pb(NO3)2 concentrations at 0, 3, 30, and 300 mg/L. Under 300 mg/L Pb(NO3)2 concentration, B. napus growth was significantly decreased, while B. juncea maintained normal growth similar to the control. The Pb accumulation was also higher in B. napus root and shoot compared to B. juncea. Gel-free proteomic analysis of roots revealed a total of 68 and 37 differentially abundant proteins (DAPs) in B. juncea and B. napus-specifically, after 300 mg/L Pb exposure. The majority of these proteins are associated with protein degradation, cellular respiration, and enzyme classification. The upregulated RPT2 and tetrapyrrole biosynthesis pathway-associated proteins maintain the cellular homeostasis and photosynthetic rate in B. juncea. Among the 55 common DAPs, S-adenosyl methionine and TCA cycle proteins were upregulated in B. juncea and down-regulated in B. napus after Pb exposure. Furthermore, higher oxidative stress also reduced the antioxidant enzyme activity in B. napus. The current finding suggests that B. juncea is more Pb tolerant than B. napus, possibly due to the upregulation of proteins involved in protein recycling, degradation, and tetrapyrrole biosynthesis pathway.

Biosynthesis of Tetrapyrrole Cofactors by Bacterial Community Inhabiting Porphyrine-Containing Shale Rock (Fore-Sudetic Monocline).

This study describes for the first time the comprehensive characterization of tetrapyrrole cofactor biosynthetic pathways developed for bacterial community (BC) inhabiting shale rock. Based on the genomic and proteomic metadata, we have detailed the biosynthesis of siroheme, heme, cobalamin, and the major precursor uroporphyrinogen III by a deep BC living on a rock containing sedimentary tetrapyrrole compounds. The obtained results showed the presence of incomplete heme and cobalamin biosynthesis pathways in the studied BC. At the same time, the production of proteins containing these cofactors, such as cytochromes, catalases and sulfite reductase, was observed. The results obtained are crucial for understanding the ecology of bacteria inhabiting shale rock, as well as their metabolism and potential impact on the biogeochemistry of these rocks. Based on the findings, we hypothesize that the bacteria may use primary or modified sedimentary porphyrins and their degradation products as precursors for synthesizing tetrapyrrole cofactors. Experimental testing of this hypothesis is of course necessary, but its evidence would point to an important and unique phenomenon of the tetrapyrrole ring cycle on Earth involving bacteria.

A tribute to Robert John Porra (august 7, 1931-may 16, 2019).

Robert John Porra (7.8.1931-16.5.2019) is probably best known for his substantial practical contributions to plant physiology and photosynthesis by addressing the problems of both the accurate spectroscopic estimation and the extractability of chlorophylls in many organisms. Physiological data and global productivity estimates, in particular of marine primary productivity, are often quoted on a chlorophyll basis. He also made his impact by work on all stages of tetrapyrrole biosynthesis: he proved the C5 pathway to chlorophylls, detected an alternative route to protoporphyrin in anaerobes and the different origin of the oxygen atoms in anaerobes and aerobes. A brief review of his work is supplemented by personal memories of the authors.

Thioredoxin-dependent control balances the metabolic activities of tetrapyrrole biosynthesis.

Plastids are specialized organelles found in plants, which are endowed with their own genomes, and differ in many respects from the intracellular compartments of organisms belonging to other kingdoms of life. They differentiate into diverse, plant organ-specific variants, and are perhaps the most versatile organelles known. Chloroplasts are the green plastids in the leaves and stems of plants, whose primary function is photosynthesis. In response to environmental changes, chloroplasts use several mechanisms to coordinate their photosynthetic activities with nuclear gene expression and other metabolic pathways. Here, we focus on a redox-based regulatory network composed of thioredoxins (TRX) and TRX-like proteins. Among multiple redox-controlled metabolic activities in chloroplasts, tetrapyrrole biosynthesis is particularly rich in TRX-dependent enzymes. This review summarizes the effects of plastid-localized reductants on several enzymes of this pathway, which have been shown to undergo dithiol-disulfide transitions. We describe the impact of TRX-dependent control on the activity, stability and interactions of these enzymes, and assess its contribution to the provision of adequate supplies of metabolic intermediates in the face of diurnal and more rapid and transient changes in light levels and other environmental factors.

Integrated analysis of microRNA and mRNA expression profiles in Crassostrea gigas to reveal functional miRNA and miRNA-targets regulating shell pigmentation.

MicroRNAs (miRNAs) regulate post-transcription gene expression by targeting genes and play crucial roles in diverse biological processes involving body color formation. However, miRNAs and miRNA-targets underlying shell color polymorphism remain largely unknown in mollusca. Using four shell colors full-sib families of the Pacific oyster Crassostrea gigas, we systematically identified miRNAs and miRNA-targets in the mantles, which organ could produce white, golden, black or partially pigmented shell. RNA sequencing and analysis identified a total of 53 known miRNA and 91 novel miRNAs, 47 of which were detected to differentially express among six pairwise groups. By integrating miRNA and mRNA expression profiles, a total of 870 genes were predicted as targets of differentially expressed miRNAs, mainly involving in biomineralization and pigmentation through functional enrichment. Furthermore, a total of four miRNAs and their target mRNAs were predicted to involve in synthesis of melanin, carotenoid or tetrapyrrole. Of them, lgi-miR-317 and its targets peroxidase and lncRNA TCONS_00951105 are implicated in acting as the competing endogenous RNA to regulate melanogenesis. Our studies revealed the systematic characterization of miRNAs profiles expressed in oyster mantle, which might facilitate understanding the intricate molecular regulation of shell color polymorphism and provide new insights into breeding research in oyster.

Roles for the chloroplast-localized pentatricopeptide repeat protein 30 and the 'mitochondrial' transcription termination factor 9 in chloroplast quality control.

Chloroplasts constantly experience photo-oxidative stress while performing photosynthesis. This is particularly true under abiotic stresses that lead to the accumulation of reactive oxygen species (ROS) which oxidize DNA, proteins and lipids. Reactive oxygen species can also act as signals to induce acclimation through chloroplast degradation, cell death and nuclear gene expression. To better understand the mechanisms behind ROS signaling from chloroplasts, we have used the Arabidopsis thaliana mutant plastid ferrochelatase two (fc2) that conditionally accumulates the ROS singlet oxygen (1 O2 ) leading to chloroplast degradation and eventually cell death. Here we have mapped mutations that suppress chloroplast degradation in the fc2 mutant and demonstrate that they affect two independent loci (PPR30 and mTERF9) encoding chloroplast proteins predicted to be involved in post-transcriptional gene expression. These mutants exhibited broadly reduced chloroplast gene expression, impaired chloroplast development and reduced chloroplast stress signaling. Levels of 1 O2 , however, could be uncoupled from chloroplast degradation, suggesting that PPR30 and mTERF9 are involved in ROS signaling pathways. In the wild-type background, ppr30 and mTERF9 mutants were also observed to be less susceptible to cell death induced by excess light stress. While broad inhibition of plastid transcription with rifampicin was also able to suppress cell death in fc2 mutants, specific reductions in plastid gene expression using other mutations was not always sufficient. Together these results suggest that plastid gene expression, or the expression of specific plastid genes by PPR30 and mTERF0, is a necessary prerequisite for chloroplasts to activate the 1 O2 signaling pathways to induce chloroplast quality control pathways and/or cell death.

Divergent Members of the Nitrogenase Superfamily: Tetrapyrrole Biosynthesis and Beyond.

The nitrogenase superfamily constitutes a large and diverse ensemble of two-component metalloenzymes. These systems couple the hydrolysis of ATP to the reduction of disparate substrates from diatomic gases (Mo and alternative nitrogenases) to photosynthetic pigments (protochlorophyllide and chlorophyllide oxidoreductases). Only very recently have the activities of the highly divergent and paraphyletic Group IV nitrogenases begun to be uncovered. This review highlights the first characterized member of this group, which was found to catalyze an unprecedented reaction in the coenzyme F430 biosynthetic pathway, and the catalytic potential of a superfamily that has yet to be fully explored.

Super-activator variants of the cyanobacterial transcriptional regulator ChlR essential for tetrapyrrole biosynthesis under low oxygen conditions.

ChlR is a MarR-type transcriptional regulator that activates the transcription of the chlAII-ho2-hemN operon in response to low oxygen conditions in the cyanobacterium Synechocystis sp. PCC 6803. Upon exposure to low oxygen conditions, ChlR activates transcription of the operon that encodes enzymes critical to tetrapyrrole biosynthesis under low oxygen conditions. We previously identified a super-activator variant, D35H, of ChlR that constitutively activates transcription of the operon. To gain insight into the low-oxygen induced activation of ChlR, we obtained eight additional super-activator variants of ChlR including D35H from pseudorevertants of a chlAI-disrupted mutant. Most substitutions were located in the N-terminal region of ChlR. Mapping of the substituted amino acid residues provided valuable structural insights that uncovered the activation mechanism of ChlR.

The retrograde signaling protein GUN1 regulates tetrapyrrole biosynthesis.

The biogenesis of the photosynthetic apparatus in developing seedlings requires the assembly of proteins encoded on both nuclear and chloroplast genomes. To coordinate this process there needs to be communication between these organelles, but the retrograde signals by which the chloroplast communicates with the nucleus at this time are still essentially unknown. The Arabidopsis thaliana genomes uncoupled (gun) mutants, that show elevated nuclear gene expression after chloroplast damage, have formed the basis of our understanding of retrograde signaling. Of the 6 reported gun mutations, 5 are in tetrapyrrole biosynthesis proteins and this has led to the development of a model for chloroplast-to-nucleus retrograde signaling in which ferrochelatase 1 (FC1)-dependent heme synthesis generates a positive signal promoting expression of photosynthesis-related genes. However, the molecular consequences of the strongest of the gun mutants, gun1, are poorly understood, preventing the development of a unifying hypothesis for chloroplast-to-nucleus signaling. Here, we show that GUN1 directly binds to heme and other porphyrins, reduces flux through the tetrapyrrole biosynthesis pathway to limit heme and protochlorophyllide synthesis, and can increase the chelatase activity of FC1. These results raise the possibility that the signaling role of GUN1 may be manifested through changes in tetrapyrrole metabolism, supporting a role for tetrapyrroles as mediators of a single biogenic chloroplast-to-nucleus retrograde signaling pathway.

Crystal structure of the first eukaryotic bilin reductase GtPEBB reveals a flipped binding mode of dihydrobiliverdin.

Phycobilins are light-harvesting pigments of cyanobacteria, red algae, and cryptophytes. The biosynthesis of phycoerythrobilin (PEB) is catalyzed by the subsequent action of two ferredoxin-dependent bilin reductases (FDBRs). Although 15,16-dihydrobiliverdin (DHBV):ferredoxin oxidoreductase (PebA) catalyzes the two-electron reduction of biliverdin IXα to 15,16-DHBV, PEB:ferredoxin oxidoreductase (PebB) reduces this intermediate further to PEB. Interestingly, marine viruses encode the FDBR PebS combining both activities within one enzyme. Although PebA and PebS share a canonical fold with similar substrate-binding pockets, the structural determinants for the stereo- and regiospecific modification of their tetrapyrrole substrates are incompletely understood, also because of the lack of a PebB structure. Here, we solved the X-ray crystal structures of both substrate-free and -bound PEBB from the cryptophyte Guillardia theta at 1.90 and 1.65 Å, respectively. The structures of PEBB exhibit the typical α/ß/α-sandwich fold. Interestingly, the open-chain tetrapyrrole substrate DHBV is bound in an unexpected flipped orientation within the canonical FDBR active site. Biochemical analyses of the WT enzyme and active site variants identified two central aspartate residues Asp-99 and Asp-219 as essential for catalytic activity. In addition, the conserved Arg-215 plays a critical role in substrate specificity, binding orientation, and active site integrity. Because these critical residues are conserved within certain FDBRs displaying A-ring reduction activity, we propose that they present a conserved mechanism for this reaction. The flipped substrate-binding mode indicates that two-electron reducing FDBRs utilize the same primary site within the binding pocket and that substrate orientation is the determinant for A- or D-ring regiospecificity.

Aerobic Enzymes and Their Radical SAM Enzyme Counterparts in Tetrapyrrole Pathways.

Microorganisms have lifestyles and metabolism adapted to environmental niches, which can be very broad or highly restricted. Molecular oxygen (O2) is currently variably present in microenvironments and has driven adaptation and microbial differentiation over the course of evolution on Earth. Obligate anaerobes use enzymes and cofactors susceptible to low levels of O2 and are restricted to O2-free environments, whereas aerobes typically take advantage of O2 as a reactant in many biochemical pathways and may require O2 for essential biochemical reactions. In this Perspective, we focus on analogous enzymes found in tetrapyrrole biosynthesis, modification, and degradation that are catalyzed by O2-sensitive radical S-adenosylmethionine (SAM) enzymes and by O2-dependent metalloenzymes. We showcase four transformations for which aerobic organisms use O2 as a cosubstrate but anaerobic organisms do not. These reactions include oxidative decarboxylation, methyl and methylene oxidation, ring formation, and ring cleavage. Furthermore, we highlight biochemically uncharacterized enzymes implicated in reactions that resemble those catalyzed by the parallel aerobic and anaerobic enzymes. Intriguingly, several of these reactions require insertion of an oxygen atom into the substrate, which in aerobic enzymes is facilitated by activation of O2 but in anaerobic organisms requires an alternative mechanism.

Ties that bind: the integration of plastid signalling pathways in plant cell metabolism.

Plastids are critical organelles in plant cells that perform diverse functions and are central to many metabolic pathways. Beyond their major roles in primary metabolism, of which their role in photosynthesis is perhaps best known, plastids contribute to the biosynthesis of phytohormones and other secondary metabolites, store critical biomolecules, and sense a range of environmental stresses. Accordingly, plastid-derived signals coordinate a host of physiological and developmental processes, often by emitting signalling molecules that regulate the expression of nuclear genes. Several excellent recent reviews have provided broad perspectives on plastid signalling pathways. In this review, we will highlight recent advances in our understanding of chloroplast signalling pathways. Our discussion focuses on new discoveries illuminating how chloroplasts determine life and death decisions in cells and on studies elucidating tetrapyrrole biosynthesis signal transduction networks. We will also examine the role of a plastid RNA helicase, ISE2, in chloroplast signalling, and scrutinize intriguing results investigating the potential role of stromules in conducting signals from the chloroplast to other cellular locations.

Chloroplast SRP43 acts as a chaperone for glutamyl-tRNA reductase, the rate-limiting enzyme in tetrapyrrole biosynthesis.

Assembly of light-harvesting complexes requires synchronization of chlorophyll (Chl) biosynthesis with biogenesis of light-harvesting Chl a/b-binding proteins (LHCPs). The chloroplast signal recognition particle (cpSRP) pathway is responsible for transport of nucleus-encoded LHCPs in the stroma of the plastid and their integration into the thylakoid membranes. Correct folding and assembly of LHCPs require the incorporation of Chls, whose biosynthesis must therefore be precisely coordinated with membrane insertion of LHCPs. How the spatiotemporal coordination between the cpSRP machinery and Chl biosynthesis is achieved is poorly understood. In this work, we demonstrate a direct interaction between cpSRP43, the chaperone that mediates LHCP targeting and insertion, and glutamyl-tRNA reductase (GluTR), a rate-limiting enzyme in tetrapyrrole biosynthesis. Concurrent deficiency for cpSRP43 and the GluTR-binding protein (GBP) additively reduces GluTR levels, indicating that cpSRP43 and GBP act nonredundantly to stabilize GluTR. The substrate-binding domain of cpSRP43 binds to the N-terminal region of GluTR, which harbors aggregation-prone motifs, and the chaperone activity of cpSRP43 efficiently prevents aggregation of these regions. Our work thus reveals a function of cpSRP43 in Chl biosynthesis and suggests a striking mechanism for posttranslational coordination of LHCP insertion with Chl biosynthesis.

5-aminolevulinic acid improves salt tolerance mediated by regulation of tetrapyrrole and proline metabolism in Brassica napus L. seedlings under NaCl stress.

5-aminolevulinic acid (ALA), a key biosynthetic precursor of tetrapyrroles, is vital for plant growth and adaptation to stress environments. Although exogenous ALA could enhance photosynthesis and biomass accumulation in plants under stress conditions, the underlying physiological and molecular mechanisms governed by ALA in promoting salt tolerance in Brassica napus L. are not yet clearly understood. In the present study, exogenous ALA with the concentration of 30 mg L-1 was applied to the leaves of B. napus seedlings subjected to 200 mM NaCl. The results showed that NaCl stress decreased the photosynthesis, biomass accumulation, and levels of chlorophyll and heme with the reduction of the concentrations of intermediates including ALA, protoporphyrin IX (Proto IX), Mg-Proto IX, and Pchlide in the tetrapyrrole (chlorophyll and heme) biosynthetic pathway. The transcript levels of genes encoding ALA-associated enzymes and genes encoding Mg-chelatase in the chlorophyll biosynthetic branch were down-regulated, while the expression levels of genes encoding Fe-chelatase in the heme branch were not significantly altered by NaCl stress. Foliar application with ALA enhanced the aboveground biomass, net photosynthetic rate, activities of antioxidant enzymes, accumulation of chlorophyll and heme, and concentrations of intermediates related to chlorophyll and heme biosynthesis in B. napus under 200 mM NaCl. The expression of most genes mentioned above remained constant in ALA-treated plants in comparison with non-ALA-treated plants under NaCl stress. Additionally, exogenous ALA synchronously induced the proline concentration and up-regulated the expression of genes P5CS and ProDH encoding proline metabolic enzymes in the NaCl treatment. These findings suggested that ALA improved salt tolerance through promoting the accumulation of chlorophyll and heme resulting from the increase of intermediate levels in the tetrapyrrole biosynthetic pathway, along with enhancing the proline accumulation in B. napus.

Investigating the bifunctionality of cyclizing and "classical" 5-aminolevulinate synthases.

The precursor to all tetrapyrroles is 5-aminolevulinic acid, which is made either via the condensation of glycine and succinyl-CoA catalyzed by an ALA synthase (the C4 or Shemin pathway) or by a pathway that uses glutamyl-tRNA as a precursor and involves other enzymes (the C5 pathway). Certain ALA synthases also catalyze the cyclization of ALA-CoA to form 2-amino-3-hydroxycyclopent-2-en-1-one. Organisms with synthases that possess this second activity nevertheless rely upon the C5 pathway to supply ALA for tetrapyrrole biosynthesis. The C5 N units are components of a variety of secondary metabolites. Here, we show that an ALA synthase used exclusively for tetrapyrrole biosynthesis is also capable of catalyzing the cyclization reaction, albeit at much lower efficiency than the dedicated cyclases. Two absolutely conserved serines present in all known ALA-CoA cyclases are threonines in all known ALA synthases, suggesting they could be important in distinguishing the functions of these enzymes. We found that purified mutant proteins having single and double substitutions of the conserved residues are not improved in their respective alternate activities; rather, they are worse. Protein structural modeling and amino acid sequence alignments were explored within the context of what is known about the reaction mechanisms of these two different types of enzymes to consider what other features are important for the two activities.

PufQ regulates porphyrin flux at the haem/bacteriochlorophyll branchpoint of tetrapyrrole biosynthesis via interactions with ferrochelatase.

Facultative phototrophs such as Rhodobacter sphaeroides can switch between heterotrophic and photosynthetic growth. This transition is governed by oxygen tension and involves the large-scale production of bacteriochlorophyll, which shares a biosynthetic pathway with haem up to protoporphyrin IX. Here, the pathways diverge with the insertion of Fe2+ or Mg2+ into protoporphyrin by ferrochelatase or magnesium chelatase, respectively. Tight regulation of this branchpoint is essential, but the mechanisms for switching between respiratory and photosynthetic growth are poorly understood. We show that PufQ governs the haem/bacteriochlorophyll switch; pufQ is found within the oxygen-regulated pufQBALMX operon encoding the reaction centre-light-harvesting photosystem complex. A pufQ deletion strain synthesises low levels of bacteriochlorophyll and accumulates the biosynthetic precursor coproporphyrinogen III; a suppressor mutant of this strain harbours a mutation in the hemH gene encoding ferrochelatase, substantially reducing ferrochelatase activity and increasing cellular bacteriochlorophyll levels. FLAG-immunoprecipitation experiments retrieve a ferrochelatase-PufQ-carotenoid complex, proposed to regulate the haem/bacteriochlorophyll branchpoint by directing porphyrin flux toward bacteriochlorophyll production under oxygen-limiting conditions. The co-location of pufQ and the photosystem genes in the same operon ensures that switching of tetrapyrrole metabolism toward bacteriochlorophyll is coordinated with the production of reaction centre and light-harvesting polypeptides.

LIL3, a Light-Harvesting Complex Protein, Links Terpenoid and Tetrapyrrole Biosynthesis in Arabidopsis thaliana.

The LIL3 protein of Arabidopsis (Arabidopsis thaliana) belongs to the light-harvesting complex (LHC) protein family, which also includes the light-harvesting chlorophyll-binding proteins of photosystems I and II, the early-light-inducible proteins, PsbS involved in nonphotochemical quenching, and the one-helix proteins and their cyanobacterial homologs designated high-light-inducible proteins. Each member of this family is characterized by one or two LHC transmembrane domains (referred to as the LHC motif) to which potential functions such as chlorophyll binding, protein interaction, and integration of interacting partners into the plastid membranes have been attributed. Initially, LIL3 was shown to interact with geranylgeranyl reductase (CHLP), an enzyme of terpene biosynthesis that supplies the hydrocarbon chain for chlorophyll and tocopherol. Here, we show another function of LIL3 for the stability of protochlorophyllide oxidoreductase (POR). Multiple protein-protein interaction analyses suggest the direct physical interaction of LIL3 with POR but not with chlorophyll synthase. Consistently, LIL3-deficient plants exhibit substantial loss of POR as well as CHLP, which is not due to defective transcription of the POR and CHLP genes but to the posttranslational modification of their protein products. Interestingly, in vitro biochemical analyses provide novel evidence that LIL3 shows high binding affinity to protochlorophyllide, the substrate of POR. Taken together, this study suggests a critical role for LIL3 in the organization of later steps in chlorophyll biosynthesis. We suggest that LIL3 associates with POR and CHLP and thus contributes to the supply of the two metabolites, chlorophyllide and phytyl pyrophosphate, required for the final step in chlorophyll a synthesis.

Elucidation of the biosynthesis of the methane catalyst coenzyme F430.

Methane biogenesis in methanogens is mediated by methyl-coenzyme M reductase, an enzyme that is also responsible for the utilization of methane through anaerobic methane oxidation. The enzyme uses an ancillary factor called coenzyme F430, a nickel-containing modified tetrapyrrole that promotes catalysis through a methyl radical/Ni(ii)-thiolate intermediate. However, it is unclear how coenzyme F430 is synthesized from the common primogenitor uroporphyrinogen iii, incorporating 11 steric centres into the macrocycle, although the pathway must involve chelation, amidation, macrocyclic ring reduction, lactamization and carbocyclic ring formation. Here we identify the proteins that catalyse the biosynthesis of coenzyme F430 from sirohydrochlorin, termed CfbA-CfbE, and demonstrate their activity. The research completes our understanding of how the repertoire of tetrapyrrole-based pigments are constructed, permitting the development of recombinant systems to use these metalloprosthetic groups more widely.

Prokaryotic Heme Biosynthesis: Multiple Pathways to a Common Essential Product.

The advent of heme during evolution allowed organisms possessing this compound to safely and efficiently carry out a variety of chemical reactions that otherwise were difficult or impossible. While it was long assumed that a single heme biosynthetic pathway existed in nature, over the past decade, it has become clear that there are three distinct pathways among prokaryotes, although all three pathways utilize a common initial core of three enzymes to produce the intermediate uroporphyrinogen III. The most ancient pathway and the only one found in the Archaea converts siroheme to protoheme via an oxygen-independent four-enzyme-step process. Bacteria utilize the initial core pathway but then add one additional common step to produce coproporphyrinogen III. Following this step, Gram-positive organisms oxidize coproporphyrinogen III to coproporphyrin III, insert iron to make coproheme, and finally decarboxylate coproheme to protoheme, whereas Gram-negative bacteria first decarboxylate coproporphyrinogen III to protoporphyrinogen IX and then oxidize this to protoporphyrin IX prior to metal insertion to make protoheme. In order to adapt to oxygen-deficient conditions, two steps in the bacterial pathways have multiple forms to accommodate oxidative reactions in an anaerobic environment. The regulation of these pathways reflects the diversity of bacterial metabolism. This diversity, along with the late recognition that three pathways exist, has significantly slowed advances in this field such that no single organism's heme synthesis pathway regulation is currently completely characterized.