Diaminopimelate decarboxylase of sporulating bacteria

A two-step transport pathway allows the mother cell to nurture the developing spore in Bacillus subtilis

One of the hallmarks of bacterial endospore formation is the accumulation of high concentrations of pyridine-2,6-dicarboxylic acid (dipicolinic acid or DPA) in the developing spore. This small molecule comprises 5–15% of the dry weight of dormant spores and plays a central role in resistance to both wet heat and desiccation. DPA is synthesized in the mother cell at a late stage in sporulation and must be translocated across two membranes (the inner and outer forespore membranes) that separate the mother cell and forespore. The enzymes that synthesize DPA and the proteins required to translocate it across the inner forespore membrane were identified over two decades ago but the factors that transport DPA across the outer forespore membrane have remained mysterious. Here, we report that SpoVV (formerly YlbJ) is the missing DPA transporter. SpoVV is produced in the mother cell during the morphological process of engulfment and specifically localizes in the outer forespore membrane. Sporulating cells lacking SpoVV produce spores with low levels of DPA and cells engineered to express SpoVV and the DPA synthase during vegetative growth accumulate high levels of DPA in the culture medium. SpoVV resembles concentrative nucleoside transporters and mutagenesis of residues predicted to form the substrate-binding pocket supports the idea that SpoVV has a similar structure and could therefore function similarly. These findings provide a simple two-step transport mechanism by which the mother cell nurtures the developing spore. DPA produced in the mother cell is first translocated into the intermembrane space by SpoVV and is then imported into the forespore by the SpoVA complex. This pathway is likely to be broadly conserved as DPA synthase, SpoVV, and SpoVA proteins can be found in virtually all endospore forming bacteria.

All pathogenic and non-pathogenic bacteria that differentiate into dormant endospores including Clostridium difficile, Bacillus anthracis, and Bacillus subtilis, contain very high concentrations of the small molecule dipicolinic acid (DPA). This molecule displaces water in the spore core where it plays an integral role in spore resistance and dormancy. DPA and its contribution to spore dehydration were discovered in 1953 but the molecular basis for its accumulation in the spore has remained unclear. The developing endospore resides within a mother cell that assembles protective layers around the spore and nurtures it by providing mother-cell-produced molecules. DPA is produced in the mother cell at a late stage in development and then must be translocated across two membranes into the spore core. Here, we report the discovery of the missing DPA transporter, homologs of which are present in virtually all endospore-forming bacteria. Our data provide evidence for a simple two-step transport pathway in which the mother cell nurtures the developing spore by sequentially moving DPA across the two membranes that surround it.

Regulation of dihydrodipicolinate synthase during growth and sporulation of Bacillus cereus

A four- to sixfold increase in specific activity of dihydrodipicolinic acid synthase was observed during sporulation of Bacillus cereus. The enzyme from cells harvested before and after the increase in specific activity appeared to be very similar as judged by pH optima, heat denaturation kinetics, apparent Michaelis constants, chromatography on diethylaminoethyl-cellulose and Sephadex G-200, and polyacrylamide gel electrophoresis. Studies with various combinations of amino acids and one of the enzyme substrates, pyruvate, failed to give evidence for control of the enzyme by activation, inhibition, repression, induction, or stabilization. Omission of calcium from the sporulation medium had no significant effect on the specific activity pattern of the enzyme as a function of age of culture.

Control of lysine biosynthesis in Bacillus subtilis: inhibition of diaminopimelate decarboxylase by lysine

Diaminopimelate decarboxylase has been characterized in extracts of Bacillus subtilis and resolved from aspartokinases I and II. Under certain conditions, the enzyme is specifically inhibited by physiological concentrations of L-lysine, but less specificity and altered kinetics of inhibition are observed if lower ionic strengths are employed in the assay procedure. Diaminopimelate decarboxylase can be desensitized to lysine inhibition by either lowering the pH or diluting the enzyme in Tris buffer in the absence of pyridoxal phosphate. Evidence is presented to incidate that, under proper conditions, lysine inhibition involves an interaction of the amino acid with the enzyme rather than competition for available pyridoxal phosphate in the assay. Lysine, by affecting the level of meso-diaminopimelate, may thus regulate its biosynthesis through sequential feedback inhibition. Analysis of the diaminopimelate decarboxylase of 15 revertants of mutants that had originally lacked diaminopimelate decarboxylase activity indicates that as little as 5% of the specific activity of enzyme observed in the wild-type strain is sufficient to permit normal growth rates. In the growing cell, diaminopimelate decarboxylase may therefore exist largely in an inhibited state.

Lysine biosynthesis in Streptomyces lipmanii: implications in antibiotic biosynthesis

Streptomyces lipmanii produces two beta-lactam antibiotics, penicillin N and 7-(5-amino-5-carboxyvaleramido)-7-methoxycephalosporanic acid. Both antibiotics contain alpha-aminoadipic acid side chains. In similar antibiotics produced by certain fungi, the alpha-aminoadipoyl moiety is derived from an intermediate in lysine biosynthesis. Our findings indicate, however, that in S. lipmanii, lysine is synthesized via alpha, epsilon-diaminopimelic acid-an entirely different biosynthetic route. This finding suggests not only a unique mechanism for the derivation of alpha-aminoadipate, but also that the system may be particularly amenable to genetic manipulation.

Isotopic study of control of the lysine biosynthetic pathway during sporulation of Bacillus cereus

The extent of incorporation of aspartate into dipicolinic acid and into various amino compounds was determined in Bacillus cereus at various times before, during, and near the end of synthesis of dipicolinic acid. The purpose of this study was to gain further information on the in vivo control of the biosynthesis of amino acids derived from aspartate. Control of the lysine biosynthetic pathway was of particular interest with regard to sporulation, owing to the important role of diaminopimelate and dipicolinate in the structure of the spore. As synthesis of dipicolinate was initiated, incorporation of carbon derived from aspartate was funneled preferentially into this compound as compared with others of the aspartate group. Incorporation into lysine essentially stopped just before the synthesis of dipicolinate began. This is consistent with the previously observed disappearance at this time of diaminopimelic acid decarboxylase in cell-free extracts. Synthesis of diaminopimelate continued during the time of synthesis of dipicolinate. The previous suggestion that diaminopimelate might exert negative control on one of the enzymes between dihydrodipicolinate and diaminopimelate is thus considered unlikely. The possibility is discussed that synthesis of dipicolinate is favored by an increase in the rate of synthesis of dihydrodipicolinate rather than by a block in its rate of utilization.

Regulation of dipicolinic acid biosynthesis in sporulating Bacillus cereus. Characterization of enzymic changes and analysis of mutants

Some of the early enzymes in the lysine-biosynthetic pathway also function for dipicolinic acid synthesis in sporulating Bacillus cereus T. 1. The first enzyme, aspartokinase, loses its sensitivity to feedback inhibition by lysing. This change occurs before the time of dipicolinic acid synthesis but at a time when diaminopimelic acid is required for spore cortex formation. 2. A possible regulatory change at a branch point in the pathway was studied by examining the properties of a key enzyme, dihydrodipicolinic acid reductase. No alteration in the feedback sensitivity or sedimentation rate of this enzyme could be detected during sporulation. 3. Two mutants producing heat-sensitive spores were analysed. Both produced spores that contained decreased amounts of dipicolinic acid. Although neither was a lysine auxotroph, they both had greatly decreased activities of certain lysine-biosynthetic enzymes in sporulating cells. 4. Starvation of cells for calcium also results in the production of spores that are heat-sensitive and contain less dipicolinic acid than the control. A decreased content of one of the lysine-biosynthetic enzymes, dihydrodipicolinic acid synthetase, in calcium-starved cells could account for the lower concentration of dipicolinic acid in the spores.

Control of diaminopimelate decarboxylase by L-lysine during growth and sporulation of Bacilluscereus

l-Lysine caused repression of diaminopimelate decarboxylase synthesis in Bacillus cereus when grown in either a minimal defined medium (CDGS medium) or a complex defined medium (a modified lysine assay medium). When cells were grown in either of the two media, variations in the specific activity of the enzyme as a function of time were found to be correlated with the intracellular lysine pool size during growth. From all of the data presented, it seems reasonable to conclude that during growth the synthesis of diaminopimelate decarboxylase is probably regulated by the intracellular lysine pool size. The relationship between lysine pool concentration and the specific activity of the enzyme did not occur in sporulating cells. The specific activity of diaminopimelate decarboxylase started to decrease at the end of exponential growth and continued to decline until it became nondetectable at the time of dipicolinic acid synthesis and development of spore refractility. Throughout this time, the intracellular lysine pool size remained below that which allowed derepression of enzyme synthesis during exponential growth. The mechanism(s) responsible for the observed decrease in the specific activity of the enzyme at the end of exponential growth is unknown. A threefold rise in the intracellular diaminopimelic acid concentration occurred when there was little or no detectable enzyme activity at the time of dipicolinic acid synthesis. This accumulation of diaminopimelic acid may exert positive control on the synthesis of spore peptidoglycan, the major component of the spore cortex.

Repression of diaminopimelate decarboxylase by L-lysine in different Bacillus species

Synthesis of diaminopimelate decarboxylase in 7 of 11 different Bacillus species tested was subject to l-lysine repression.

Cell wall polymers of Bacillus sphaericus 9602. II. Synthesis of the first enzyme unique to cortex synthesis during sporulation

The cell wall peptidoglycan of vegetative cells of Bacillus sphaericus 9602 contains l-lysine and d-isoasparagine and is devoid of diaminopimelic acid (Dap), whereas the peptidoglycan of its spore cortex is devoid of l-lysine and d-isoasparagine and contains meso-Dap. These two structures have a common biosynthetic precursor, uridine-diphospho-N-acetylmuramyl-l- alanyl-d-glutamic acid, which accepts either l-lysine or meso-Dap, the latter reaction being the first unique to the synthesis of the spore cortex peptidoglycan. l-lysine-adding activity decays at the end of vegetative growth to a level which is maintained until Dap-adding activity appears, when it declines rapidly again. Dap-adding activity is not detectable in refractile spores, in vegetative cells, or in sporulating cells until about 4 hr after the end of vegetative growth, when it increases rapidly for about 1.5 hr in a process dependent on continued protein and ribonucleic acid (RNA) synthesis. This process apparently involves transcription and translation during this period of a "sporulation-specific" gene whose product is essential for and unique to sporulation. It is closely followed by the acquirement of refractility. Another sporulation-specific gene, that for dipicolinate synthase, is apparently transcribed and translated in an overlapping period commencing about 0.5 hr later, although dipicolinate does not accumulate rapidly until 1.5 hr later, when about 75% of the cells are already refractile. Inhibition of protein synthesis with chloramphenicol or of RNA synthesis with streptolydigin inhibited accumulation of these enzymes in sporulating cells; this inhibition could be reversed by washing out the antibiotics after 1.5 hr. Sporulation recommenced with an unaltered sequence of events but with poorer synchrony. There was no evidence for a messenger RNA for either enzyme of lifetime greater than a small fraction of the period of enzyme accumulation, although dilution with 10 volumes of fresh medium failed to prevent synthesis of Dap-adding enzyme in cells which had become terminally swollen, a process preceding enzyme synthesis by about 1.5 hr. The synthesis of this enzyme in B. sphaericus is apparently dependent on programmed transcription of the appropriate gene.