Occurrence and properes of two types of delta-aminolevulinate synthetase in Rhodopseudomonas spheroides

5-Aminolevulinic acid synthesis in epimastigotes of Trypanosoma cruzi

BACKGROUND AND AIMS

Trypanosoma cruzi is the causative agent of Chagas disease or American trypanosomiasis. The parasite manifests a nutritional requirement for heme compounds because of its biosynthesis deficiency. The aim of this study has been to investigate the presence of metabolites and enzymes of porphyrin pathway, as well as ALA formation in epimastigotes of T. cruzi, Tulahuén strain, Tul 2 stock.

METHODS

Succinyl CoA synthetase, 5-aminolevulinic acid (ALA) synthetase, 4,5-dioxovaleric (DOVA) transaminase, ALA dehydratase and porphobilinogenase activities, as well as ALA, porphobilinogen (PBG), free porphyrins and heme content were measured in a parasite cells-free extract. Extracellular content of these metabolites was also determined.

RESULTS

DOVA, PBG, porphyrins and heme were not detected in acellular extracts of T. cruzi. However ALA was detected both intra- and extracellularly This is the first time that the presence of ALA (98% of intracellularly formed ALA) is demonstrated in the extracellular medium of a parasite culture. Regarding the ALA synthesizing enzymes, DOVA transaminase levels found were low (7.13+/-0.49EU/mg protein), whilst ALA synthetase (ALA-S) activity was undetectable. A compound of non-protein nature, low molecular weight, heat unstable, inhibiting bacterial ALA-S activity was detected in an acellular extract of T. cruzi. This inhibitor could not be identified with either ALA, DOVA or heme.

CONCLUSIONS

ALA synthesis is functional in the parasite and it would be regulated by the heme levels, both directly and through the inhibitor factor detected. ALA formed can not be metabolized further, because the necessary enzymes are not active, therefore it should be excreted to avoid intracellular cytotoxicity.

Characterization of the rhodobacter sphaeroides 5-aminolaevulinic acid synthase isoenzymes, HemA and HemT, isolated from recombinant Escherichia coli

The hemA and hemT genes encoding 5-aminolaevulinic acid synthase (ALAS) from the photosynthetic bacterium Rhodobacter sphaeroides, were cloned to allow high expression in Escherichia coli. Both HemA and HemT appeared to be active in vivo as plasmids carrying the respective genes complemented an E. coli hemA strain (glutamyl-tRNA reductase deficient). The over-expressed isoenzymes were isolated and purified to homogeneity. Isolated HemA was soluble and catalytically active whereas HemT was largely insoluble and failed to show any activity ex vivo. Pure HemA was recovered in yields of 5-7 mg x L-1 of starting bacterial culture and pure HemT at 10 mg x L-1 x HemA has a final specific activity of 13 U x mg-1 with 1 unit defined as 1 micromol of 5-aminolaevulinic acid formed per hour at 37 degrees C. The Km values for HemA are 1.9 mM for glycine and 17 microM for succinyl-CoA, with the enzyme showing a turnover number of 430 h-1. In common with other ALASs the recombinant R. sphaeroides HemA requires pyridoxal 5'-phosphate (PLP) as a cofactor for catalysis. Removal of this cofactor resulted in inactive apo-ALAS. Similarly, reduction of the HemA-PLP complex using sodium borohydride led to > 90% inactivation of the enzyme. Ultraviolet-visible spectroscopy with HemA suggested the presence of an aldimine linkage between the enzyme and pyridoxal 5'-phosphate that was not observed when HemT was incubated with the cofactor. HemA was found to be sensitive to reagents that modify histidine, arginine and cysteine amino acid residues and the enzyme was also highly sensitive to tryptic cleavage between Arg151 and Ser152 in the presence or absence of PLP and substrates. Antibodies were raised to both HemA and HemT but the respective antisera were not only found to bind both enzymes but also to cross-react with mouse ALAS, indicating that all of the proteins have conserved epitopes.

Regulation of 5-aminolevulinic acid synthesis in Rhodobacter sphaeroides 2.4.1: the genetic basis of mutant H-5 auxotrophy

Rhodobacter sphaeroides H-5 was isolated as a 5-aminolevulinic acid (ALA) auxotroph following treatment of wild-type cells with N-methyl-N-nitroso-N'-nitroguanidine (J. Lascelles and T. Altshuler, J. Bacteriol. 98:721-727, 1969). The existence in R. sphaeroides 2.4.1 of the genes hemA and hemT, each encoding the enzyme 5-aminolevulinic acid synthase (EC 2.3.1.37), raised questions as to the genetic basis for the ALA auxotrophy in mutant H-5. We therefore cloned both the hemA and hemT genes from mutant H-5. The hemA gene has been sequenced in its entirety and bears four base pair substitutions which encode three amino acid changes relative to the sequence of wild-type strain 2.4.1. Complementation analysis of an Escherichia coli ALA auxotroph has revealed that the loss of ALA synthase activity in the HemA mutant enzyme could be localized to two of the amino acid substitutions. On the other hand, the hemT gene from mutant H-5 was able to complement an E. coli mutant requiring ALA for growth. Complementation analyses were also carried out by introducing the cloned hemA or hemT gene of mutant H-5 or wild-type 2.4.1 in trans into H-5 and, in parallel, into our previously described HemA-HemT double mutant strain AT1 (E. L. Neidle and S. Kaplan, J. Bacteriol. 175:2304-2313, 1993). This analysis revealed that while the complementation pattern of mutant AT1 parallels that for the E. coli ALA auxotroph, mutant H-5 could only be complemented by the wild-type hemA gene. The ability of the hemT gene of either mutant H-5 or wild-type 2.4.1 to complement the ALA auxotrophy of mutant AT1 but not mutant H-5 was consistent with beta-galactosidase activities obtained with hemT-lacZ transcriptional fusions. We conclude that the ALA auxotrophy of mutant H-5 arises from (i) a nonfunctional HemA protein containing multiple missense substitutions and (ii) an inability of the normal hemT gene to be expressed in the mutant H-5 genetic background, i.e., an additional mutation of unknown origin is required for hemT expression. These studies bear directly on the regulation of the expression of the hemA and hemT genes of R. sphaeroides 2.4.1.

A putative anaerobic coproporphyrinogen III oxidase in Rhodobacter sphaeroides. I. Molecular cloning, transposon mutagenesis and sequence analysis of the gene

A mutant of Rhodobacter sphaeroides, N1, has been isolated which is incapable of photosynthetic growth and, instead of synthesizing bacteriochlorophyll, N1 excretes coproporphyrin III into the growth medium. Using conjugative gene transfer, several clones were isolated from a R. sphaeroides gene library which restored normal pigment synthesis and photosynthetic growth to N1. Using transposon Tn5 mutagenesis, the gene was located to a 1.05 kb EcoRI fragment. Sequence and transcription analysis defined the position and expression of an open reading frame of approximately 920 bp, which is proposed as the anaerobic coproporphyrinogen III oxidase dedicated to bacteriochlorophyll biosynthesis.

Heme-deficient mutants of Salmonella typhimurium: two genes required for ALA synthesis

The first step in heme biosynthesis is the formation of 5-aminolevulinic acid (ALA). We have isolated, mapped and characterized a large number of Salmonella typhimurium mutants auxotrophic for ALA. These mutants carry defects in either one of two genes, both required for ALA synthesis. The previously identified hemA gene maps at 35 min, and the hemL gene maps at 5 min on the S. typhimurium genetic map. Mutants in hemA and hemL are defective for aerobic and anaerobic respiration, and appear to be oxygen sensitive. The Hem- phenotype of hemL mutants is less severe than that of hemA mutants. Although hemA and hemL mutants are deficient in heme synthesis, genetic tests indicate that they still synthesize two minor products of the heme pathway, siroheme and cobalamin (vitamin B12), under anaerobic conditions. In contrast, hemB, hemC and cysG mutants, blocked after ALA synthesis, make neither siroheme nor vitamin B12. Double mutants defective in both hemA and hemL also make siroheme. We suggest that hemA and hemL are required for one route of ALA synthesis and that a second, minor route of ALA synthesis may operate in S. typhimurium; this second pathway would be independent of the hemA and hemL functions.

Cloning and characterization of the 5-aminolevulinate synthase gene(s) from Rhodobacter sphaeroides

The 5-aminolevulinate synthase gene (hemA) from Rhizobium meliloti was used to probe a genomic lambda bank derived from Rhodobacter sphaeroides DNA. Two phage clones were found to bear homology to the Rhizobium probe. Southern hybridization analysis of the two lambda phage clones, which we designated lambda Hem 10 and lambda Hem 12, showed that the homology to the Rhizobium hemA gene was localized to a 3.1-kb SalI fragment derived from lambda Hem 10 and a 7.0-kb SalI fragment derived from lambda Hem 12. Each of the SalI fragments was subsequently cloned into the multiple cloning site of pUC19 in both orientations relative to the lac promoter. Restriction analysis confirmed that each SalI fragment was unique. It was also shown from Southern hybridization analysis that the regions of homology within each of the R. sphaeroides restriction fragments and the Rhizobium probe were different. Further, we have tentatively concluded that each R. sphaeroides hemA gene shows a relatively low degree of homology to the other. Data obtained from in vitro transcription-translation studies in a homologous R. sphaeroides cell-free system, and complementation of hemA mutations of both Escherichia coli and R. sphaeroides by either of the putative hemA clones suggested the presence of a gene encoding 5-aminolevulinate synthase on each DNA sequence. The fact that 5-aminolevulinate synthase activity could be demonstrated in mutant strains complemented in trans with either cloned DNA fragment further supported this conclusion.

Inhibition of bacteriochlorophyll synthesis in Rhodobacter sphaeroides subsp. denitrificans grown in light under denitrifying conditions

The inclusion of nitrate or nitrite in cultures of Rhodobacter spaeroides subsp. denitrificans grown heterotrophically in light depressed the formation of bacteriochlorophyll a. The pigment biosynthesis was inhibited at the stage of the reduction of chlorophyllide (chlorin) to bacteriochlorophyllide (tetrahydroporphyrin) since 3-hydroxyethylchlorophyllide a accumulated in the culture medium. The addition of exogenous 5-aminolevulinic acid to these cultures resulted in a complete restoration of bacteriochlorophyll synthesis accompanied by the accumulation of 3-vinylbacteriopheophorbide. This indicates that under these conditions bacteriochlorophyll was formed via an alternative route, in which the reduction of chlorins to tetrahydroporphyrins precedes modifications of the C-3 side chain. The multiple forms of 5-aminolevulinic acid synthase were purified from cells grown with and without nitrate. Antibodies against these proteins were raised in rabbits and used in enzyme-linked immunosorbent assays for various forms of 5-aminolevulinic acid synthase. In denitrifying cells, the amount and activity of fraction I of the enzyme was reduced by approximately 40 and 30%, respectively. Partly active enzymes from both types of cells were activated by cystine trisulfide.

Tetrapyrrole biosynthesis: N-methyl-N'-nitrosoguanidine-induced mutants of Propionibacterium shermanii

An isolation method forN-methyl-N′-nitrosoguanidine-induced catalase negative mutants ofP. shermaniibased on replica plating is described. In contrast to previous methods, it extends to the early stages of tetrapyrrole biosynthesis which are common in both corrins and porphyrins. It may thus aid in elucidating the mechanism and control of porphyrin and corrin biosynthesis. Some preliminary results are discussed.

5-Aminolevulinic-acid synthetases from Rhodopseudomonas spheroides Y. Comparison of the purification and properties of enzymes extracted from bacteria grown in different iron concentrations

The two 5-aminolevulinic acid synthetases of Rhodopseudomonas spheroides Y. were extracted from cells grown in a 'low-iron' medium and purified. They have a specific activity 10-fold higher than the 'high-iron' enzymes described by us previously and have the same properties except that they do not contain any iron and have one free-SH group more per mole of enzyme (2 for E1; , for E2) Their inhibition by adenosine triphosphate and iron and their oxidation-reduction sensitivity are discussed in terms of light, oxygen and heme feed-back regulation of bacteriochlorophyll sunthesis.

Control of 5-aminolaevulinate synthetase activity in Rhodopseudomonas spheroides a role for trisulphides

1. The aminolaevulinate synthetase activator of fresh extracts of semi-anaerobically grown Rhodopseudomonas spheroids was resolved into two fractions by ion-exchange chromatography. One fraction was identified as cystine trisulphide (CySSSCy). Analysis of the other fraction indicated the presence of about equal amounts of glutathione trisulphide (GSSSG) and the mixed trisulphide of glutathione and cystine (GSSSCy). 2. Four further fractions with activator activity were observed on ion-exchange chromatography of extracts prepared by methods similar to those described earlier [Neuberger et al. (1973)Biochem. J. 136,491-499]. These activators were generated by the extraction procedure. Two of them have been identified as trisulphanedisulphonate (S5O62-) and additional cystine trisulphide. 3. For the series of polysulphanedisulphonates (-O3S-Sn-SO3-, n greater than or equal to 1), activator activity at muM concentrations was exhibited only by compounds with n greater than 3. This, together with the results described above, indicates that for a compound R-Sn-R' (where R and R' are organic or inorganic groups) the only structural requirement for activity is n greater than or equal to 3. 4. Oxygenation of a semipanaerobic culture of R. spheroids for 1.5h before harvesting the cells produced a decrease of more than 90% in the cellular content of cystine trisulphide and glutathione trisulphides. 5. Chromatography on DEAE-Sephadex confirmed the presence of multiple forms of aminolaevulinate synthetase in extracts of R. spheroides [Tuboi et al. (1970) Arch.Biochem. Biophys. 138,147-154]. Oxygenation of a semi-anaerobic culture resulted in the disappearance of high-activity enzyme (a-form) and the accumulation of low-activity enzyme (b-form) in the cell. Spontaneous activation [Marriott et al. (1969) Biochem. J. 111,385-394] And activation by cystine trisulphide both resulted in the almost complete conversion of the b-form into the a-form.

Mechanism and stereochemistry of the 5-aminolaevulinate synthetase reaction

1. Two mechanisms for the biosynthesis of 5-aminolaevulinate from glycine and succinyl-CoA (3-carboxypropionyl-CoA) are considered. One of the mechanisms involves the retention of both the C-2 H atoms of glycine during the synthesis of 5-aminolaevulinate, whereas the other predicts the retention of only one of the C-2 H atoms of glycine. 2. Highly purified 5-aminolaevulinate synthetase from Rhodopseudomonas spheroides was used to show that the C-2 H atom of glycine with R configuration is specifically removed during the biosynthesis of 5-aminolaevulinate. 3. The mechanism of the condensation therefore differs from the analogous reaction of the biosynthesis of sphinganine from palmitoyl-CoA and serine, in which the C-2 H of serine is retained (Wiess, 1963).