5-Aminolevulinic acid synthesis in Escherichia coli

Production of 5-aminolevulinic acid from hydrolysates of cassava residue and fish waste by engineered Bacillus cereus PT1

The economical production of 5-aminolevulinic acid (ALA) has recently received increasing attention for its extensive use in agriculture. In this study, a strain of Bacillus cereus PT1 could initially produce ALA at a titre of 251.72 mg/L by using a hydrolysate mixture of low-cost cassava residue and fish waste. The integration of endogenous hemA encoding glutamyl-tRNA reductase led to a 39.30% increase in ALA production. Moreover, improving cell permeability by deletion of the LytR-CpsA-Psr (LCP) family gene tagU led to a further increase of 59.73% in ALA production. Finally, the engineered strain B. cereus PT1-hemA-ΔtagU produced 2.62 g/L of ALA from the previously mentioned hydrolysate mixture in a 7-L bioreactor. In a pot experiment, foliar spray of the ALA produced by B. cereus PT1-hemA-ΔtagU from the hydrolysates increased salt tolerance of cucumber by improving chlorophyll content and catalase activity, while decreasing malondialdehyde content. Overall, this study demonstrated an economic way to produce ALA using a microbial platform and evidenced the potential of ALA in agricultural application.

Strain engineering for high-level 5-aminolevulinic acid production in Escherichia coli

Herein, we report the development of a microbial bioprocess for high-level production of 5-aminolevulinic acid (5-ALA), a valuable non-proteinogenic amino acid with multiple applications in medical, agricultural, and food industries, using Escherichia coli as a cell factory. We first implemented the Shemin (i.e., C4) pathway for heterologous 5-ALA biosynthesis in E. coli. To reduce, but not to abolish, the carbon flux toward essential tetrapyrrole/porphyrin biosynthesis, we applied clustered regularly interspersed short palindromic repeats interference (CRISPRi) to repress hemB expression, leading to extracellular 5-ALA accumulation. We then applied metabolic engineering strategies to direct more dissimilated carbon flux toward the key precursor of succinyl-CoA for enhanced 5-ALA biosynthesis. Using these engineered E. coli strains for bioreactor cultivation, we successfully demonstrated high-level 5-ALA biosynthesis from glycerol (~30 g L^-1 ) under both microaerobic and aerobic conditions, achieving up to 5.95 g L^-1 (36.9% of the theoretical maximum yield) and 6.93 g L^-1 (50.9% of the theoretical maximum yield) 5-ALA, respectively. This study represents one of the most effective bio-based production of 5-ALA from a structurally unrelated carbon to date, highlighting the importance of integrated strain engineering and bioprocessing strategies to enhance bio-based production.

Production of 5-aminolevulinic acid from glutamate by overexpressing HemA1 and pgr7 from Arabidopsis thaliana in Escherichia coli

The important metabolic intermediate 5-aminolevulinic acid (ALA) is useful for cancer treatment or plant growth regulation and has consequently received much attention. In this study, we introduced the HemA1 and pgr7 genes from the higher plant Arabidopsis thaliana into recombinant Escherichia coli to overproduce extracellular 5-aminolevulinic acid via the C5 pathway. In the E. coli BL21 (DE3) strain background, the ALA concentration of the strain expressing both HemA1 and pgr7 was the highest and reached 3080.62 mg/L. Among the 7 tested hosts, ALA production was the highest in E. coli Transetta (DE3). In E. coli Transetta GTR/GBP, the expression levels of zwf, gnd, pgl and RhtA were upregulated. Glutamate induced the expression of the GltJ, GltK, GltL and GltS genes that are in involved in glutamate uptake. The recombinant E. coli Transetta GTR/GBP was able to produce 7642 mg/L ALA in modified minimal medium supplemented with 10 g/L glutamate and 15 g/L glucose after 48 h of fermentation at 22 °C. The results provide persuading evidence for the efficient production of ALA from glucose and glutamate in E. coli expressing A. thaliana HemA1 and pgr7. Further optimization of the fermentation process should be done to improve the ALA production to an industrially relevant level.

Structural studies of domain movement in active-site mutants of porphobilinogen deaminase from Bacillus megaterium

The enzyme porphobilinogen deaminase (PBGD) is one of the key enzymes in tetrapyrrole biosynthesis. It catalyses the formation of a linear tetrapyrrole from four molecules of the substrate porphobilinogen (PBG). It has a dipyrromethane cofactor (DPM) in the active site which is covalently linked to a conserved cysteine residue through a thioether bridge. The substrate molecules are linked to the cofactor in a stepwise head-to-tail manner during the reaction, which is catalysed by a conserved aspartate residue: Asp82 in the B. megaterium enzyme. Three mutations have been made affecting Asp82 (D82A, D82E and D82N) and their crystal structures have been determined at resolutions of 2.7, 1.8 and 1.9 Å, respectively. These structures reveal that whilst the D82E mutant possesses the DPM cofactor, in the D82N and D82A mutants the cofactor is likely to be missing, incompletely assembled or disordered. Comparison of the mutant PBGD structures with that of the wild-type enzyme shows that there are significant domain movements and suggests that the enzyme adopts `open' and `closed' conformations, potentially in response to substrate binding.

Biosynthesis and Use of Cobalamin (B12)

This review summarizes research performed over the last 23 years on the genetics, enzyme structures and functions, and regulation of the expression of the genes encoding functions involved in adenosylcobalamin (AdoCbl, or coenzyme B12) biosynthesis. It also discusses the role of coenzyme B12 in the physiology of Salmonella enterica serovar Typhimurium LT2 and Escherichia coli. John Roth's seminal contributions to the field of coenzyme B12 biosynthesis research brought the power of classical and molecular genetic, biochemical, and structural approaches to bear on the extremely challenging problem of dissecting the steps of what has turned out to be one of the most complex biosynthetic pathways known. In E. coli and serovar Typhimurium, uro'gen III represents the first branch point in the pathway, where the routes for cobalamin and siroheme synthesis diverge from that for heme synthesis. The cobalamin biosynthetic pathway in P. denitrificans was the first to be elucidated, but it was soon realized that there are at least two routes for cobalamin biosynthesis, representing aerobic and anaerobic variations. The expression of the AdoCbl biosynthetic operon is complex and is modulated at different levels. At the transcriptional level, a sensor response regulator protein activates the transcription of the operon in response to 1,2-Pdl in the environment. Serovar Typhimurium and E. coli use ethanolamine as a source of carbon, nitrogen, and energy. In addition, and unlike E. coli, serovar Typhimurium can also grow on 1,2-Pdl as the sole source of carbon and energy.

5-Aminolevulinic acid production in engineered Corynebacterium glutamicum via C5 biosynthesis pathway

ALA (5-aminolevulinic acid) is an important intermediate in the synthesis of tetrapyrroles and the use of ALA has been gradually increasing in many fields, including medicine and agriculture. In this study, improved biological production of ALA in Corynebacterium glutamicum was achieved by overexpressing glutamate-initiated C5 pathway. For this purpose, copies of the glutamyl t-RNA reductase HemA from several bacteria were mutated by site-directed mutagenesis of which a HemA version from Salmonella typhimurium exhibited the highest ALA production. Cultivation of the HemA-expressing strain produced approximately 204 mg/L of ALA, while co-expression with HemL (glutamate-1-semialdehyde aminotransferase) increased ALA concentration to 457 mg/L, representing 11.6- and 25.9-fold increases over the control strain (17 mg/L of ALA). Further effects of metabolic perturbation were investigated, leading to penicillin addition that further improves ALA production to 584 mg/L. In an optimized flask fermentation, engineered C. glutamicum strains expressing the HemA and hemAL operon produced up to 1.1 and 2.2g/L ALA, respectively, under glutamate-producing conditions. The final yields represent 10.7- and 22.0-fold increases over the control strain (0.1g/L of ALA). From these findings, ALA biosynthesis from glucose was successfully demonstrated and this study is the first to report ALA overproduction in C. glutamicum via metabolic engineering.

Effect of hydrogen peroxide on the biosynthesis of heme and proteins: potential implications for the partitioning of Glu-tRNA(Glu) between these pathways

Glutamyl-tRNA (Glu-tRNA(Glu)) is the common substrate for both protein translation and heme biosynthesis via the C5 pathway. Under normal conditions, an adequate supply of this aminoacyl-tRNA is available to both pathways. However, under certain circumstances, Glu-tRNA(Glu) can become scarce, resulting in competition between the two pathways for this aminoacyl-tRNA. In Acidithiobacillus ferrooxidans, glutamyl-tRNA synthetase 1 (GluRS1) is the main enzyme that synthesizes Glu-tRNA(Glu). Previous studies have shown that GluRS1 is inactivated in vitro by hydrogen peroxide (H2O2). This raises the question as to whether H2O2 negatively affects in vivo GluRS1 activity in A. ferrooxidans and whether Glu-tRNA(Glu) distribution between the heme and protein biosynthesis processes may be affected by these conditions. To address this issue, we measured GluRS1 activity. We determined that GluRS1 is inactivated when cells are exposed to H2O2, with a concomitant reduction in intracellular heme level. The effects of H2O2 on the activity of purified glutamyl-tRNA reductase (GluTR), the key enzyme for heme biosynthesis, and on the elongation factor Tu (EF-Tu) were also measured. While exposing purified GluTR, the first enzyme of heme biosynthesis, to H2O2 resulted in its inactivation, the binding of glutamyl-tRNA to EF-Tu was not affected. Taken together, these data suggest that in A. ferrooxidans, the flow of glutamyl-tRNA is diverted from heme biosynthesis towards protein synthesis under oxidative stress conditions.

Recent advances in microbial production of δ-aminolevulinic acid and vitamin B12

δ-aminolevulinate (ALA) is an important intermediate involved in tetrapyrrole synthesis (precursor for vitamin B12, chlorophyll and heme) in vivo. It has been widely applied in agriculture and medicine. On account of many disadvantages of its chemical synthesis, microbial production of ALA has been received much attention as an alternative because of less expensive raw materials, low pollution, and high productivity. Vitamin B12, one of ALA derivatives, which plays a vital role in prevention of anaemia has also attracted intensive works. In this review, recent advances on the production of ALA and vitamin B12 with novel approaches such as whole-cell enzyme-transformation and metabolic engineering are described. Furthermore, the direction for future research and perspective are also summarized.

Functional asymmetry for the active sites of linked 5-aminolevulinate synthase and 8-amino-7-oxononanoate synthase

5-Aminolevulinate synthase (ALAS) and 8-amino-7-oxononanoate synthase (AONS) are homodimeric members of the α-oxoamine synthase family of pyridoxal 5'-phosphate (PLP)-dependent enzymes. Previously, linking two ALAS subunits into a single polypeptide chain dimer yielded an enzyme (ALAS/ALAS) with a significantly greater turnover number than that of wild-type ALAS. To examine the contribution of each active site to the enzymatic activity of ALAS/ALAS, the catalytic lysine, which also covalently binds the PLP cofactor, was substituted with alanine in one of the active sites. Albeit the chemical rate for the pre-steady-state burst of ALA formation was identical in both active sites of ALAS/ALAS, the k(cat) values of the variants differed significantly (4.4±0.2 vs. 21.6±0.7 min(-1)) depending on which of the two active sites harbored the mutation. We propose that the functional asymmetry for the active sites of ALAS/ALAS stems from linking the enzyme subunits and the introduced intermolecular strain alters the protein conformational flexibility and rates of product release. Moreover, active site functional asymmetry extends to chimeric ALAS/AONS proteins, which while having a different oligomeric state, exhibit different rates of product release from the two ALAS and two AONS active sites due to the created intermolecular strain.

Evidence for an inter-organismic heme biosynthetic pathway in symbiotic soybean root nodules

The successful symbiosis of soybean with Bradyrhizobium japonicum depends on their complex interactions, culminating in the development and maintenance of root nodules. A B. japonicum mutant defective in heme synthesis in culture was able to produce heme as a result of its symbiotic association with the soybean host. The bacterial mutant was incapable of synthesizing the committed heme precursor delta-aminolevulinic acid (ALA), but nodule plant cells formed ALA from glutamate. In addition, exogenous ALA was taken up by isolated nodule bacteria of the parent strain and of the mutant. It is proposed that bacterial heme found in nodules can be synthesized from plant ALA, hence segments of a single metabolic pathway are spatially separated into two organisms.

Biochemistry, regulation and genomics of haem biosynthesis in prokaryotes

Haems are involved in many cellular processes in prokaryotes and eukaryotes. The biosynthetic pathway leading to haem formation is, with few exceptions, well-conserved, and is controlled in accordance with cellular function. Here, we review the biosynthesis of haem and its regulation in prokaryotes. In addition, we focus on a modification of haem for cytochrome c biogenesis, a complex process that entails both transport between cellular compartments and a specific thioether linkage between the haem moiety and the apoprotein. Finally, a whole genome analysis from 63 prokaryotes indicates intriguing exceptions to the universality of the haem biosynthetic pathway and helps define new frontiers for future study.

Circular permutation of 5-aminolevulinate synthase. Mapping the polypeptide chain to its function

5-Aminolevulinate synthase is the first enzyme of the heme biosynthetic pathway in non-plant eukaryotes and some prokaryotes. The enzyme functions as a homodimer and requires pyridoxal 5'-phosphate as a cofactor. Although the roles of defined amino acids in the active site and catalytic mechanism have been recently explored using site-directed mutagenesis, much less is known about the role of the 5-aminolevulinate synthase polypeptide chain arrangement in folding, structure, and ultimately, function. To assess the importance of the continuity of the polypeptide chain, circularly permuted 5-aminolevulinate synthase variants were constructed through either rational design or screening of an engineered random library. One percent of the random library clones were active, and a total of 21 active variants had sequences different from that of the wild type 5-aminolevulinate synthase. Out of these 21 variants, 9 displayed unique circular permutations of the 5-aminolevulinate synthase polypeptide chain. The new termini of the active variants disrupted secondary structure elements and loop regions and fell in 100 amino acid regions from each terminus. This indicates that the natural continuity of the 5-aminolevulinate synthase polypeptide chain and the sequential arrangement of the secondary structure elements are not requirements for proper folding, binding of the cofactor, or assembly of the two subunits. Furthermore, the order of two identified functional elements (i.e. the catalytic and the glycine-binding domains) is apparently irrelevant for proper functioning of the enzyme. Although the wild type 5-aminolevulinate synthase and the circularly permuted variants appear to have similar, predicted overall tertiary structures, they exhibit differences in the arrangement of the secondary structure elements and in the cofactor-binding site environment. Taken together, the data lead us to propose that the 5-aminolevulinate synthase overall structure can be reached through multiple or alternative folding pathways.

Role of the hemA gene product and delta-aminolevulinic acid in regulation of Escherichia coli heme synthesis

We initiated these studies to help clarify the roles of heme, delta-aminolevulinic acid (ALA), hemA, and hemM in Escherichia coli heme synthesis. Using recombinant human hemoglobin (rHb1.1) as a tool for increasing E. coli's heme requirements, we demonstrated that heme is a feedback inhibitor of heme synthesis. Cooverexpression of rHb1.1 and the hemA-encoded glutamyl-tRNA (GTR) reductase increased intracellular levels of ALA and heme and increased the rate of rHb1.1 formation. These results support the conclusion that heme synthesis is limited by ALA (S. Hino and A. Ishida, Enzyme 16:42-49, 1973; W. K. Philipp-Dormston and M. Doss, Enzyme 16:57-64, 1973) and that the hemA-encoded GTR reductase is a rate-limiting enzyme in the pathway (J.-M. Li, C. S. Russell, and S. D. Cosloy, Gene 82:2099-217, 1989). Increasing the copy number of hemM, whose product is believed to be required for efficient ALA formation (W. Chen, C. S. Russell, Y. Murooka, and S. D. Cosloy, J. Bacteriol. 176:2743-2746, 1994; M. Ikemi, K. Murakami, M. Hashimoto, and Y. Murooka, Gene 121:127-132, 1992), had no effect on either ALA pools or the rate of rHb1.1 accumulation. The hemA-encoded GTR reductase was found to be regulated by ALA. Some of our results differ from those reported by Hart and coworkers (R. A. Hart, P. T. Kallio, and J. E. Bailey, Appl. Environ. Microbiol. 60:2431-2437, 1994), who concluded that ALA formation is not the rate-limiting step in E. coli cells expressing Vitreoscilla hemoglobin.

Regulation of heme biosynthesis in Salmonella typhimurium: activity of glutamyl-tRNA reductase (HemA) is greatly elevated during heme limitation by a mechanism which increases abundance of the protein

In Salmonella typhimurium and Escherichia coli, the hemA gene encodes the enzyme glutamyl-tRNA reductase, which catalyzes the first committed step in heme biosynthesis. We report that when heme limitation is imposed on cultures of S. typhimurium, glutamyl-tRNA reductase (HemA) enzyme activity is increased 10- to 25-fold. Heme limitation was achieved by a complete starvation for heme in hemB, hemE, and hemH mutants or during exponential growth of a hemL mutant in the absence of heme supplementation. Equivalent results were obtained by both methods. To determine the basis for this induction, we developed a panel of monoclonal antibodies reactive with HemA, which can detect the small amount of protein present in a wild-type strain. Western blot (immunoblot) analysis with these antibodies reveals that the increase in HemA enzyme activity during heme limitation is mediated by an increase in the abundance of the HemA protein. Increased HemA protein levels were also observed in heme-limited cells of a hemL mutant in two different E. coli backgrounds, suggesting that the observed regulation is conserved between E. coli and S. typhimurium. In S. typhimurium, the increase in HemA enzyme and protein levels was accompanied by a minimal (less than twofold) increase in the expression of hemA-lac operon fusions; thus HemA regulation is mediated either at a posttranscriptional step or through modulation of protein stability.

Glutamate-1-semialdehyde aminotransferase from Sulfolobus solfataricus

Glutamate-1-semialdehyde aminotransferase (GSA-AT) from the extremely thermophilic bacterium Sulfolobus solfataricus has been purified to homogeneity and characterized. GSA-AT is the last enzyme in the C5 pathway for the conversion of glutamate into the tetrapyrrole precursor delta-aminolaevulinate (ALA) in plants, algae and several bacteria. The active form of GSA-AT from S. solfataricus seems to be a homodimer with a molecular mass of 87 kDa. The absorption spectrum of the purified aminotransferase is indicative of the presence of pyridoxamine 5'-phosphate (PMP) cofactor, and the catalytic activity of the enzyme is further stimulated by addition of PMP. 3-Amino-2,3-dihydrobenzoic acid is an inhibitor of the aminotransferase activity. The N-terminal amino acid sequence of GSA-AT from S. solfataricus was found to share significant similarity with the eukaryotic and eubacterial enzymes. Evidence is provided that ALA synthesis in S. solfataricus follows the C5 pathway characteristic of plants, algae, cyanobacteria and many other bacteria.

Expression of glutamyl-tRNA reductase in Escherichia coli

The biosynthesis of the hemes, chlorophylls, corrins and other tetrapyrroles begins with the synthesis of 5-aminolevulinic acid (ALA). The pathway is highly conserved except for the synthesis of ALA which is derived from glycine and succinyl CoA (C4) in most eukaryotes and from glutamate (C5) in most bacteria and in green plants. In C5, glutamyl-tRNA synthetase (GTS) converts glutamate to glutamyl-tRNA (glu-tRNA), which is reduced by glutamyl-tRNA reductase (GTR) to glutamyl-1-semialdehyde (GSA), which is converted by aminotransferase (GSA-AT) to ALA. Since GTS is also involved in protein synthesis and GSA can be converted to ALA non-enzymatically, it is highly probable that control of ALA synthesis and thus of the whole pathway resides in the GTR step. In Escherichia coli, GTR is the gene product of hemA. BL21(DE3), a protease-deficient strain which contains the T7 RNA polymerase gene in front of a lac promoter, was transformed with a pET14b-based vector, pWC01, harboring hemA in front of a T7 promoter and ORF1 which is transcribed in the opposite direction. The transformed strain, WC1201, secreted ALA and porphyrins into the medium. Induction of expression of hemA by WC1201 was optimized for concentration of inducer (IPTG, 5 mM), temperature (37 degrees C), presence of betaine and sorbitol (no change) and time of induction (2h). GTR was observable as a 46 kDa band by Brilliant blue G staining of SDS-PAGE gels. Sonicates of the induction mixture exhibited strong ALA synthesis activity which was enhanced by tRNAglu. Most of the activity was in the supernatant of the sonicate indicating that GTR is a soluble enzyme. The induced strain had more GTS activity than the uninduced strain which had more GTS activity than its parent wild-type strain. Autoradiography on native gradient PAGE showed that GTR expressed in vivo by induction of WC1201 had a molecular weight of approx. 117 kDa. Gel filtration of the induced sonicate showed a peak of enzymatic activity at about 126 kDa. When pET14b- or pUC19-based plasmids harboring hemA and ORF1, or importantly, a pUC19-based plasmid harboring only hemA and not ORF1, were expressed in an in vitro transcription-translation system, native gradient PAGE showed a product with a molecular weight of approximately 175 kDA. This expression was higher in the presence of tRNAglu. When the 117 kDa and 175 kDa proteins were excised from their native gels respectively, and run on SDS PAGE, autoradiography showed bands at 46 kDa. We conclude that GTR is present in both high molecular weight species. Since overexpression of hemA from pET14b-based plasmids is associated with increased glutamyl-tRNA synthetase activity, the 175 kDa species may represent different complexes of GTR, GTS and glutamyl-tRNA as observed in Chlamydomonas and the 117-126 kDa species may be an dimer of GTR associated with glu-tRNA or a complex of GTR, GTS and glu-tRNA. These possibilities are being investigated.

5-Aminolevulinate production by Escherichia coli containing the Rhodobacter sphaeroides hemA gene

The Rhodobacter sphaeroides hemA gene codes for 5-aminolevulinate (ALA) synthase. This enzyme catalyzes the pyridoxal phosphate-dependent condensation of succinyl coenzyme A and glycine-forming ALA. The R. sphaeroides hemA gene in the pUC18/19 vector system was transformed into Escherichia coli. The effects of both genetic and physiological factors on the expression of ALA synthase and the production of ALA were studied. ALA synthase activity levels were maximal when hemA had the same transcription direction as the lac promoter. The distance between the lac promoter and hemA affected the expression of ALA synthase on different growth substrates. The E. coli host strain used had an enormous effect on the ALA synthase activity level and on the production of ALA, with E. coli DH1 being best suited. The ALA synthase activity level was also dependent on the carbon source. Succinate, L-malate, fumarate, and L-aspartate gave the highest levels of ALA synthase activity, while the use of lactose as a carbon source resulted in a repression of ALA synthase. After growth on succinate, ALA synthase represented approximately 5% of total cellular protein. The ALA synthase activity level was also dependent on the pH of the medium, with maximal activity occurring at pH 6.5. ALA production by whole cells was limited by the availability of glycine, and the addition of 2 g of glycine per liter to the growth medium increased the production of ALA fivefold, to 2.25 mM. In recombinant E. coli extracts, up to 22 mM ALA was produced from succinate, glycine, and ATP.

Expression of catalytically active barley glutamyl tRNAGlu reductase in Escherichia coli as a fusion protein with glutathione S-transferase

delta-Aminolevulinate in plants, algae, cyanobacteria, and several other bacteria such as Escherichia coli and Bacillus subtilis is synthesized from glutamate by means of a tRNA(Glu) mediated pathway. The enzyme glutamyl tRNA(Glu) reductase catalyzes the second step in this pathway, the reduction of tRNA bound glutamate to give glutamate 1-semialdehyde. The hemA gene from barley encoding the glutamyl tRNA(Glu) reductase was expressed in E. coli cells joined at its amino terminal end to Schistosoma japonicum glutathione S-transferase (GST). GST-glutamyl tRNA(Glu) reductase fusion protein and the reductase released from it by thrombin digestion catalyzed the reduction of glutamyl tRNA(Glu) to glutamate 1-semialdehyde. The specific activity of the fusion protein was 120 pmol.micrograms-1.min-1. The fusion protein used tRNA(Glu) from barley chloroplasts preferentially to E. coli tRNA(Glu) and its activity was inhibited by hemin. It migrated as an 82-kDa polypeptide with SDS/PAGE and eluted with an apparent molecular mass of 450 kDa from Superose 12. After removal of the GST by thrombin, the protein migrated as an approximately equal to 60-kDa polypeptide with SDS/PAGE, whereas gel filtration on Superose 12 yielded an apparent molecule mass of 250 kDa. Isolated fusion protein contained heme, which could be reduced by NADPH and oxidized by air.

Transcription of the glutamyl-tRNA reductase (hemA) gene in Salmonella typhimurium and Escherichia coli: role of the hemA P1 promoter and the arcA gene product

In Salmonella typhimurium and Escherichia coli, the hemA gene encodes the enzyme glutamyl-tRNA reductase, which catalyzes the first committed step in the heme biosynthetic pathway. It has recently been reported that a lac operon fusion to the hemA promoter of E. coli is induced 20-fold after starvation for heme. Induction was dependent on the transcriptional regulator ArcA, with a second transcriptional regulator, FNR, playing a negative role specifically under anaerobic conditions (S. Darie and R. P. Gunsalus, J. Bacteriol. 176:5270-5276, 1994). We have investigated the generality of this effect by examining the response to heme starvation of a number of lac operon fusions to the hemA promoters of both E. coli and S. typhimurium. We confirmed that such fusions are induced during starvation of a hemA auxotroph, but the level of induction observed was maximally sixfold and for S. typhimurium fusions it was only two- to fourfold. Sequences required for high-level expression of hemA lie within 129 bp upstream of the major (P1) promoter transcriptional start site. Mutants defective in the P1 promoter had greatly reduced hemA-lac expression both in the presence and in the absence of ALA. Mutations in arcA had no effect on hemA-lac expression in E. coli during normal growth, although the increase in expression during starvation for ALA was half that seen in an arcA+ strain. Overexpression of the arcA gene had no effect on hemA-lac expression. Primer extension analysis showed that RNA 5' ends mapping to the hemA P1 and P2 promoters were not expressed at significantly higher levels in induced cultures. These results differ from those previously reported.

Partial inhibition of protein synthesis accelerates the synthesis of porphyrin in heme-deficient mutants of Escherichia coli

Mutants of Escherichia coli defective in the HemA protein grow extremely poorly as the result of heme deficiency. A novel hemA mutant was identified whose rate of growth was dramatically enhanced by addition to the medium of low concentrations of translational inhibitors, such as chloramphenicol and tetracycline. This mutant (H110) carries mutation at position 314 in the hemA gene, which resulted in diminished activity of the encoded protein. Restoration of growth of H110 upon addition of the drugs mentioned above was due to activation of the synthesis of porphyrin. However, this activation was not characteristic exclusively of cells with this mutant hemA gene since it was also observed in a heme-deficient strain bearing the wild-type hemA gene. The activation did not depend on the promoter activity of the hemA gene, as indicated by studies with fusion genes. It appears that partial inhibition of protein synthesis via inhibition of peptidyltransferase can promote the synthesis of porphyrin by providing an increased supply of glutamyl-tRNA for porphyrin synthesis. Glutamyl-tRNA is the common substrate for peptidyltransferase and HemA.

The common origins of the pigments of life-early steps of chlorophyll biosynthesis

The complex pathway of tetrapyrrole biosynthesis can be dissected into five sections: the pathways that produce 5-aminolevulinate (the C-4 and the C-5 pathways), the steps that transform ALA to uroporphyrinogen III, which are ubiquitous in the biosynthesis of all tetrapyrroles, and the three branches producing specialized end products. These end products include corrins and siroheme, chlorophylls and hemes and linear tetrapyrroles. These branches have been subjects of recent reviews. This review concentrates on the early steps leading up to uroporphyrinogen III formation which have been investigated intensively in recent years in animals, in plants, and in a wide range of bacteria.

Expression of a biologically active plant cytochrome b5 in Escherichia coli

Cytochrome b5 from tobacco (Nicotiana tabacum) was expressed in Escherichia coli using a T7 polymerase/promoter system as described by Studier, Rosenberg, Dunn and Dubendorff (1990) (Methods Enzymol. 185, 60-89). Transformed cells were red in colour and accumulated cytochrome b5 to a level of around 30% of the total cell protein. The purified cytochrome had oxidized, reduced and low-temperature absorbance spectra characteristic of plant microsomal cytochrome b5, and exhibited a c.d. spectrum resembling that of a mammalian cytochrome b5. The recombinant protein appeared to be correctly assembled and biologically active, being reduced by NADH in the presence of microsomal membranes prepared from the developing seeds of sunflower (Helianthus annuus). Inhibition of haem synthesis in the transformed E. coli cells expressing cytochrome b5, by the use of gabaculin or succinylacetone, prevented the assembly of the cytochrome b5 holoprotein but had little effect on the accumulation of cytochrome apoprotein. The recombinant protein expressed in E. coli therefore has the biochemical features of the higher-plant cytochrome b5 and can be used in studies of plant microsomal oxidation/reduction reactions.

Effect of heme and oxygen availability on hemA gene expression in Escherichia coli: role of the fnr, arcA, and himA gene products

While many organisms synthesize delta-aminolevulinate, the precursor of heme, by condensing succinyl-coenzyme A and glycine, others use a glutamate-dependent pathway in which glutamyl-tRNA dehydrogenase catalyzes the rate-determining step. The hemeA gene that encodes this latter enzyme in Escherichia coli has been cloned and sequenced. To examine how its expression is regulated, we constructed hemA-lacZ operon and gene fusions and inserted them into the chromosome in single copy. The effect of aerobic and anaerobic growth conditions and the availability of electron acceptors and various carbon substrates were documented. Use of different types of cell culture medium resulted in a fivefold variation in hemA-lacZ expression during aerobic cell growth. Anaerobic growth resulted in 2.5-fold-higher hemA-lacZ expression than aerobic growth. This control is mediated by the fnr and arcA gene products. Fnr functions as a repressor of hemA transcription during anaerobic cell growth only, whereas the arcA gene product activates hemA gene expression under both aerobic and anaerobic conditions. Integration host factor protein was also shown to be required for control of hemA gene regulation. To determine whether an intermediate or a product of the heme biosynthetic pathway is involved in hemA regulation, hemA-lacZ expression was analyzed in a hemA mutant. Expression was elevated by 20-fold compared with that in a wild-type strain, while the addition of the heme pathway intermediate delta-aminolevulinate to the culture medium restored expression to wild-type levels. These results suggest that the heme pathway is feedback regulated at the level of hemA gene expression, to supply heme as it is required during different modes of cell growth.

A gene cluster inChlorobium vibrioforme encoding the first enzymes of chlorophyll biosynthesis

A cloned 5.8-kb genomic fragment of the green sulfur bacteriumChlorobium vibrioforme encodes the genes for three enzymes catalyzing early steps in the biosynthetic pathway of tetrapyrroles, common to chlorophyll and heme. ThehemA, hemC andhemD genes encode the enzymes glutamyl tRNA dehydrogenase, porphobilinogen deaminase and uroporphyrinogen III synthase, respectively. The cloned genes were expressed in transformedEscherichia coli orSalmonella typhimurium and conferred autotrophy on the respective auxotrophs. Activities of the enzymes encoded by the cloned genes were demonstrated in vitro, with cell extracts obtained from the transformed enterobacteria. The proximity of these genes indicates that they form a cluster inChlorobium vibrioforme, while in most other organisms they appear to be scattered. The presence of this cluster may imply coordinate regulation of the genes involved and they may constitute an operon.

Effect of biosynthetic manipulation of heme on insolubility of Vitreoscilla hemoglobin in Escherichia coli

Vitreoscilla hemoglobin (VHb) is accumulated at high levels in both soluble and insoluble forms when expressed from its native promoter on a pUC19-derived plasmid in Escherichia coli. Examination by atomic absorption spectroscopy and electron paramagnetic resonance spectroscopy revealed that the insoluble form uniformly lacks the heme prosthetic group (apoVHb). The purified soluble form contains heme (holoVHb) and is spectroscopically indistinguishable from holoVHb produced by Vitreoscilla cells. This observation suggested that a relationship may exist between the insolubility of apoVHb and biosynthesis of heme. To examine this possibility, a series of experiments were conducted to chemically and genetically manipulate the formation and conversion of 5-aminolevulinic acid (ALA), a key intermediate in heme biosynthesis. Chemical perturbations involved supplementing the growth medium with the intermediate ALA and the competitive inhibitor levulinic acid which freely cross the cell barrier. Genetic manipulations involved amplifying the gene dosage for the enzymes ALA synthase and ALA dehydratase. Results from both levulinic acid and ALA supplementations indicate that the level of soluble holoVHb correlates with the heme level but that the level of insoluble apoVHb does not. The ratio of soluble to insoluble VHb also does not correlate with the level of total VHb accumulated. The effect of amplifying ALA synthase and ALA dehydratase gene dosage is complex and may involve secondary factors. Results indicate that the rate-limiting step of heme biosynthesis in cells overproducing VHb does not lie at ALA synthesis, as it reportedly does in wild-type E. coli (S. Hino and A. Ishida, Enzyme 16:42-49, 1973).

5-Aminolevulinic acid synthesis in Escherichia coli requires expression of hemA

hemA and hemM, which are 213 bp apart and divergently transcribed, were separately cloned. We found that hemA is required for 5-aminolevulinic acid (ALA) synthesis in two ALA- auxotrophs. Overexpression of hemM alone did not produce ALA. More ALA was produced by strains harboring a plasmid with both hemA and hemM than by those with hemA alone. We conclude that hemA alone is required for ALA synthesis but hemA and hemM are required for maximal ALA synthesis.

Cloning and characterization of the hemA gene for synthesis of delta-aminolevulinic acid in Xanthomonas campestris pv. phaseoli

The gene from Xanthomonas campestris pv. phaseoli that is involved in the C5 pathway of delta-amino-levulinic acid (ALA) of Escherichia coli. Subcloning of deletion fragments from the initial 2.5-kilobase (kb) chromosomal fragment allowed the isolation of a 1.6-kb fragment that could complement the hemM mutation. Nucleotide sequence analysis of the 1.6-kb DNA fragment revealed an open reading frame that encodes a polypeptide of 426 amino acid residues, and the deduced molecular mass of this polypeptide is 46768 Da. The amino acid sequence shows a high degree of homology of the HemA protein, which is glutamyl-tRNA reductase, to other organisms. Thus, we examined the complementation test of the cloned gene from Xanthomonas with a hemA mutation of E. coli and found that the gene complemented the hemA mutation. These results suggest that the cloned gene is hemA and the gene from Xanthomonas also complements both hemA and hemM mutations, as in the case of the E. coli hemA.

Sequence and molecular analysis of the nifL gene of Azotobacter vinelandii

In both Klebsiella pneumoniae and Azotobacter vinelandii the nifL gene, which encodes a negative regulator of nitrogen fixation, lies immediately upstream of nifA. We have sequenced the A. vinelandii nifL gene and found that it is more homologous in its C-terminal domain to the histidine protein kinases (HPKs) than is K. pneumoniae NifL. In particular A. vinelandii NifL contains a conserved histidine at a position shown to be phosphorylated in other systems. Both NifL proteins are homologous in their N-termini to a part of the Halobacterium halobium bat gene product; Bat is involved in regulation of bacterio-opsin, the expression of which is oxygen sensitive. The same region showed homology to the haem-binding N-terminal domain of the Rhizobium meliloti fixL gene product, an oxygen-sensing protein. Like K. pneumoniae NifL, A. vinelandii NifL is shown here to prevent expression of nif genes in the presence of NH+4 or oxygen. The sequences found homologous in the C-terminal regions of NifL, FixL and Bat might therefore be involved in oxygen binding or sensing. An in-frame deletion mutation in the nifL coding region resulted in loss of repression by NH+4 and the mutant excreted high amounts of ammonia during nitrogen fixation, thus confirming a phenotype reported earlier for an insertion mutation. In addition, nifLA are cotranscribed in A. vinelandii as in K. pneumoniae, but expression from the A. vinelandii promoter requires neither RpoN nor NtrC.

Expression of the soybean (Glycine max) glutamate 1-semialdehyde aminotransferase gene in symbiotic root nodules

Extracts of soybean (Glycine max) root nodules and greening etiolated leaves catalyzed radiolabeled delta-aminolevulinic acid (ALA) formation from 3,4-[3H]glutamate but not from 1-[14C]glutamate. Nevertheless, those tissue extracts expressed the activity of glutamate 1-semialdehyde (GSA) aminotransferase, the C5 pathway enzyme that catalyzes ALA synthesis from GSA for tetrapyrrole formation. A soybean nodule cDNA clone that conferred ALA prototrophy, GSA aminotransferase activity, and glutamate-dependent ALA formation activity on an Escherichia coli GSA aminotransferase mutant was isolated. The deduced product of the nodule cDNA shared 79% identity with the GSA aminotransferase expressed in barley leaves, providing, along with the complementation data, strong evidence that the cDNA encodes GSA aminotransferase. GSA aminotransferase mRNA and enzyme activity were expressed in nodules but not in uninfected roots, indicating that the Gsa gene is induced in the symbiotic tissue. The Gsa gene was strongly expressed in leaves of etiolated plantlets independently of light treatment and, to a much lesser extent, in leaves of mature plants. We conclude that GSA aminotransferase, and possibly the C5 pathway, is expressed in a nonphotosynthetic plant organ for nodule heme synthesis and that Gsa is a regulated gene in soybean.

Nitrogen regulation in an Escherichia coli strain with a temperature sensitive glutamyl-tRNA synthetase

Escherichia coli cells carrying the gltX351 allele are unable to grow at 42 degrees C (Ts phenotype) due to an altered glutamyl-tRNA synthetase. We found that gltX351 cells display a new phenotype termed Gsd-, i.e. an inability to raise glutamine synthetase activity above low constitutive levels in minimal medium with 6.8 mM glutamine as sole nitrogen source. When 0.5 mM NH4+ or 12 mM glutamate replaced glutamine, the glutamine synthetase activities of gltX351 cells were raised to wild-type levels. Northern experiments showed that the Gsd- phenotype is the result of an impairment in transcription initiation from the Ntr-regulated promoter, glnAp2. Intragenic and extragenic secondary mutations appeared frequently in gltX351 cells, which suppressed their Gsd- but not their Ts phenotype. Moreover, in heterozygous gltX+/gltX351 partial diploids, gltX351 was dominant for the Gsd- phenotype and recessive for the Tr phenotype. A slight increase in the glutamine pool and in the intracellular glutamine: 2-oxoglutarate ratio was also observed but this could not account for the Gsd- phenotype of gltX351 cells. In cells carrying gltX351 and a suppressor of the Gsd- phenotype, sup-1, tightly linked to gltX351, the glutamine pool and glutamine: 2-oxoglutarate intracellular ratio were even higher than in the gltX351 single mutant. These results indicate that the gltX351 mutant polypeptide may be the direct cause of the Gsd- phenotype. The possibility that it interacts with one or more components that trigger the Ntr response is discussed.

The Escherichia coli visA gene encodes ferrochelatase, the final enzyme of the heme biosynthetic pathway

An Escherichia coli mutant with a disrupted visA gene was defective in ferrochelatase activity but expressed wild-type levels of protoporphyrinogen oxidase activity. The visA coding region was placed under the transcriptional control of T7 RNA polymerase in an E. coli expression system, and the product was expressed as a 38-kDa protein. The overexpressed protein was purified to near homogeneity and was found to contain ferrochelatase activity. The data show that the visA gene encodes ferrochelatase, and we propose that it be renamed hemH to reflect that conclusion.

The genes required for heme synthesis in Salmonella typhimurium include those encoding alternative functions for aerobic and anaerobic coproporphyrinogen oxidation

Insertion mutagenesis has been used to isolate Salmonella typhimurium strains that are blocked in the conversion of 5-aminolevulinic acid (ALA) to heme. These mutants define the steps of the heme biosynthetic pathway after ALA. Insertions were recovered at five unlinked loci: hemB, hemCD, and hemE, which have been mapped previously in S. typhimurium, and hemG and hemH, which have been described only for Escherichia coli. No other simple hem mutants were found. However, double mutants are described that are auxotrophic for heme during aerobic growth and fail to convert coproporphyrinogen III to protoporphyrinogen IX. These mutant strains are defective in two genes, hemN and hemF. Single mutants defective only in hemN require heme for anaerobic growth on glycerol plus nitrate but not for aerobic growth on glycerol. Mutants defective only in hemF have no apparent growth defect. We suggest that these two genes encode alternative forms of coproporphyrinogen oxidase. Anaerobic heme synthesis requires hemN function, while either hemN or hemF is sufficient for aerobic heme synthesis. These phenotypes are consistent with the requirement of a well-characterized class of coproporphyrinogen oxidase for molecular oxygen.

Characterization of delta-Aminolevulinic Acid Formation in Soybean Root Nodules

Formation of the heme precursor delta-aminolevulinic acid (ALA) was studied in soybean root nodules elicited by Bradyrhizobium japonicum. Glutamate-dependent ALA formation activity by soybean (Glycine max) in nodules was maximal at pH 6.5 to 7.0 and at 55 to 60 degrees C. A low level of the plant activity was detected in uninfected roots and was 50-fold greater in nodules from 17-day-old plants; this apparent stimulation correlated with increases in both plant and bacterial hemes in nodules compared with the respective asymbiotic cells. The glutamate-dependent ALA formation activity was greatest in nodules from 17-day-old plants and decreased by about one-half in those from 38-day-old plants. Unlike the eukaryotic ALA formation activity, B. japonicum ALA synthase activity was not significantly different in nodules than in cultured cells, and the symbiotic activity was independent of nodule age. The lack of symbiotic induction of B. japonicum ALA synthase indicates either that ALA formation is not rate-limiting, or that ALA synthase is not the only source of ALA for bacterial heme synthesis in nodules. Plant cytosol from nodules catalyzed the formation of radiolabeled ALA from U-[(14)C]glutamate and 3,4-[(3)H]glutamate but not from 1-[(14)C]glutamate, and thus, operation of the C(5) pathway could not be confirmed.

Glutamyl-tRNA reductase activity in Bacillus subtilis is dependent on the hemA gene product

The Bacillus subtilis hemAXCDBL operon encodes enzymes for the synthesis of 5-aminolaevuline acid via the C5 pathway (hemA and hemL) and uroporphyrinogen III (hemB, hemC and hemD). B. subtilis HemA protein (molecular mass 50 kDa) was overexpressed in hemA mutant of both Escherichia coli and B. subtilis. A mutant B. subtilis HemA protein with a Cys to Tyr change at position 105 was also overexpressed. Both wild-type and mutant HemA proteins migrated as oligomers (molecular mass greater than or equal to 230 kDa) on gel-filtration columns. All column fractions containing wild-type HemA protein had glutamyl-tRNA reductase activity. No glutamyl-tRNA reductase activity was found with the mutant HemA protein. It is concluded that the B. subtilis hemA gene product is identical to, or part of, the glutamyl-tRNA reductase of the C5 pathway.

Enzymatic basis of thiol-stimulated secretion of porphyrins by Escherichia coli

1-Thioglycerol (TG) stimulates the synthesis of porphyrin in aerobically growing Escherichia coli. Here the levels of delta-aminolevulinate biosynthetic enzymes in untreated and TG-treated E. coli THU and PUC2 (a mutant of THU which overproduces porphyrins in the presence of thiols) cells were determined. TG treatment elevated the activity of glutamyl-tRNA reductase in both strains. The increased activity was not caused by activation of preexisting enzymes by thiols or by oxidizing agents but was dependent on new protein synthesis.

Photosensitivity of a protoporphyrin-accumulating, light-sensitive mutant (visA) of Escherichia coli K-12

Mutations in the visA gene of Escherichia coli cause the mutant bacteria to die upon illumination with visible light. We confirmed genetically that the visA gene is a structural gene for ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1). Since other mutations in the genes involved in the biosynthesis of heme can cure the photosensitivity, the light-induced cell death appears to be brought about by the accumulation of protoporphyrin IX, one of the substrates of ferrochelatase. When cells are illuminated with visible light, protoporphyrin IX seems to produce an active species of oxygen (probably 1O2) that is harmful to the cells. This defect is the same as that associated with the human disease protoporphyria.

Structure and expression of the Chlorobium vibrioforme hemA gene

The green sulfur bacterium, Chlorobium vibrioforme, synthesizes the tetrapyrrole precursor, delta-aminolevulinic acid (ALA), from glutamate via the RNA-dependent five-carbon pathway. A 1.9-kb clone of genomic DNA from C. vibrioforme that is capable of transforming a glutamyl-tRNA reductase-deficient, ALA-dependent, hemA mutant of Escherichia coli to prototrophy was sequenced. The transforming C. vibrioforme DNA has significant sequence similarity to the E. coli, Salmonella typhimurium, and Bacillus subtilis hemA genes and contains a 1245 base open reading frame that encodes a 415 amino acid polypeptide with a calculated molecular weight of 46174. This polypeptide has over 28% amino acid identity with the polypeptides deduced from the nucleic acid sequences of the E. coli, S. typhimurium, and B. subtilis hemA genes. No sequence similarity was detected, at either the nucleic acid or the peptide level, with the Rhodobacter capsulatus or Bradyrhizobium japonicum hemA genes, which encode ALA synthase, or with the S. typhimurium hemL gene, which encodes glutamate-1-semialdehyde aminotransferase. These results establish that hemA encodes glutamyl-tRNA reductase in species that use the five-carbon ALA biosynthetic pathway. A second region of the cloned DNA, located downstream from the hemA gene, has significant sequence similarity to the E. coli and B. subtilis hemC genes. This region contains a potential open reading frame that encodes a polypeptide that has high sequence identity to the deduced E. coli and B. subtilis HemC peptides. hemC encodes the tetrapyrrole biosynthetic enzyme, porphobilinogen deaminase, in these species.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriochlorophyll c formation via the C5 pathway of 5-aminolevulinic acid synthesis in Chloroflexus aurantiacus

Biosynthesis of 5-aminolevulinic acid (ALA) in Chloroflexus aurantiacus, a thermophilic bacterium forming bacteriochlorophyll c, is shown to proceed via the C5 pathway by demonstrating (1) the specific labeling of its chlorin ring with [1 - 13C]glutamate and (2) the enzyme activity to produce ALA from glutamate in a cell-free extract. From the phylogenetic distribution it is suggested that ALA synthetase distributed in some aerobic eubacteria could be monophyletic in origin.

Cloning and nucleotide sequence of the hemA gene of Agrobacterium radiobacter

The hemA gene of Agrobacterium radiobacter ATCC4718 was identified by hybridization with a hemA probe from Rhizobium meliloti and cloned by complementation of a hemA mutant of Escherichia coli K12. E. coli hemA transformants carrying the hemA gene of Agrobacterium showed delta-aminolevulinic acid synthetase (delta-ALAS) activity in vitro. The hemA gene was carried on a 4.4 kb EcoRI fragment which could be reduced to a 2.6 kb EcoRI-SstI fragment without affecting its complementing or delta-ALAS activity. The sequence of the hemA gene showed an open reading frame of 1215 nucleotides, which could code for a protein of 44,361 Da. This is very close to the molecular weight of the HemA protein obtained using an in vitro coupled transcription-translation system (45,000 Da). Comparison of amino acid sequences of the delta-ALAS of A. radiobacter and Bradyrhizobium japonicum showed strong homology between the two enzymes; less, but still significant, homology was observed when A. radiobacter and human delta-ALAS were compared. Primer extension experiments enabled us to identify two promoters for the hemA gene of A. radiobacter. One of these promoters shows some similarity to the first promoter of the hemA gene of R. meliloti.

Aerobic growth and respiration of a delta-aminolevulinic acid synthase (hemA) mutant of Bradyrhizobium japonicum

Oxygen-dependent growth of the Bradyrhizobium japonicum hemA mutant MLG1 (M.L. Guerinot and B.K. Chelm, Proc. Natl. Acad. Sci. USA 83:1837-1841, 1986) was demonstrated in cultured cells in the absence of exogenous delta-aminolevulinic acid (ALA), but growth of analogous mutants of Rhizobium meliloti or of Escherichia coli was not observed unless ALA was added to the yeast extract-containing media. No heme could be detected in extracts of strain MLG1 cells as measured by the absorption or by the peroxidase activity of the heme moiety, but the rates of growth and endogenous respiration of the mutant were essentially identical to those found in the parent strain. A role for ALA in the viability of strain MLG1 could not be ruled out since the ALA analog levulinic acid inhibited growth, but neither ALA synthase nor glutamate-dependent ALA synthesis activity was found in the mutant. The data show that the cytochromes normally discerned in wild-type B. japonicum cultured cells by absorption spectroscopy are not essential for aerobic growth or respiration.

delta-Aminolevulinic acid dehydratase deficiency can cause delta-aminolevulinate auxotrophy in Escherichia coli

Ethylmethane sulfonate-induced mutants of several Escherichia coli strains that required delta-aminolevulinic acid (ALA) for growth were isolated by penicillin enrichment or by selection for respiratory-defective strains resistant to the aminoglycoside antibiotic kanamycin. Three classes of mutants were obtained. Two-thirds of the strains were mutants in hemA. Representative of a third of the mutations was the hem-201 mutation. This mutation was mapped to min 8.6 to 8.7. Complementation of the auxotrophic phenotype by wild-type DNA from the corresponding phage 8F10 allowed the isolation of the gene. DNA sequence analysis revealed that the hem-201 gene encoded ALA dehydratase and was similar to a known hemB gene of E. coli. Complementation studies of hem-201 and hemB1 mutant strains with various hem-201 gene subfragments showed that hem-201 and the previously reported hemB1 mutation are in the same gene and that no other gene is required to complement the hem-201 mutant. ALA-forming activity from glutamate could not be detected by in vitro or in vivo assays. Extracts of hem-201 cells had drastically reduced ALA dehydratase levels, while cells transformed with the plasmid-encoded wild-type gene possessed highly elevated enzyme levels. The ALA requirement for growth, the lack of any ALA-forming enzymatic activity, and greatly reduced ALA dehydratase activity of the hem-201 strain suggest that a diffusible product of an enzyme in the heme biosynthetic pathway after ALA formation is involved in positive regulation of ALA biosynthesis. In contrast to the hem-201 mutant, previously isolated hemB mutants were not ALA auxotrophs and had no detectable ALA dehydratase activity.(ABSTRACT TRUNCATED AT 250 WORDS)