Changes in patterns of alkaline serine protease and bacilysin formation caused by common effectors of sporulation inBacillus subtilis 168

Biosynthesis, Molecular Regulation, and Application of Bacilysin Produced by Bacillus Species

Microbes produce a diverse range of secondary metabolites in response to various environmental factors and interspecies competition. This enables them to become superior in a particular environment. Bacilysin, a dipeptide antibiotic produced by Bacillus species, is active against a broad range of microorganisms. Because of its simple structure and excellent mode of action, i.e., through the inhibition of glucosamine 6-phosphate synthase, it has drawn the attention of researchers. In addition, it acts as a pleiotropic signaling molecule that affects different cellular activities. However, all Bacillus species are not capable of producing bacilysin. The biosynthesis of bacilysin by Bacillus species is not uniform throughout the population; specificity and heterogeneity at both the strain and species levels has been observed. This review discusses how bacilysin is biosynthesized by Bacillus species, the regulators of its biosynthesis, its importance in the host, and the abiotic factors affecting bacilysin production.

Biochemistry, genetics and regulation of bacilysin biosynthesis and its significance more than an antibiotic

Bacillus subtilis has the capacity to produce more than two dozen bioactive compounds with an amazing variety of chemical structures. Among them, bacilysin is a non-ribosomally synthesized dipeptide antibiotic consisting of l-alanine residue at the N terminus and a non-proteinogenic amino acid, l-anticapsin, at the C terminus. In spite of its simple structure, it is active against a wide range of bacteria and fungi. As a potent antimicrobial agent, we briefly review the biochemistry and genetics as well as the regulation of bacilysin biosynthesis within the frame of peptide pheromones-based control of secondary activities. Biological functions of bacilysin in the producer B. subtilis beyond its antimicrobial activity as well as potential biotechnological use of the biosynthetic enzyme l-amino acid ligase (Lal) are also discussed.

Global regulatory systems operating in Bacilysin biosynthesis in Bacillus subtilis

In Bacillus subtilis, bacilysin is a nonribosomally synthesized dipeptide antibiotic composed of L-alanine and L-anticapsin. The biosynthesis of bacilysin depends on the bacABCDEywfG operon (bac operon)and the adjacent ywfH gene. To elucidate the effects of global regulatory genes on the expression of bac operon, we used the combination of lacZ fusion analysis and the gel mobility shift assays. The cell density-dependent transition state induction of the bac operon was clearly shown. The basal expression level of the bac operon as well as transition state induction of bac is directly ComA dependent. Three Phr peptides, PhrC, PhrF and PhrK, are required for full-level expression of ComA-dependent bac operon expression, but the most important role seemed to be played by PhrC in stimulating bac expression through a RapC-independent manner. Spo0A is another positive regulator which participates in the transition state induction of bac both directly by interacting with the bac promoter and indirectly by repressing abrB expression. AbrB and CodY proteins do not only directly repress the bac promoter, but they also mutually stimulate the transition state induction of bac indirectly, most likely by antagonizing their repressive effects without preventing each other's binding since both proteins can bind to the bac promoter simultaneously.

The novel gene yvfI in Bacillus subtilis is essential for bacilysin biosynthesis

Using transposon mutagenesis in Bacillus subtilis PY79, three independent mutants defective in production of bacilysin were isolated. To identify the genes in these mutant loci affecting bacilysin biosynthesis, the inserted transposon and its flanking regions were cloned and sequenced from each mutant. Transposon insertions in these three mutants were found to be in the yvfI gene which encodes an unknown protein similar to GntR family transcriptional regulators. For further confirmation, deletion mutants were constructed in which nucleotides 196-314 of the yvfI gene were removed. All resulting yvfI (Delta196-314)::spc deletion mutants exhibited bacilysin-negative phenotypes, as in the case of the yvfI::Tn10::spc insertional mutants. The lacR gene, encoding a transcriptional regulator, resides immediately downstream from the yvfI gene. Therefore, an insertion mutation was created in the lacR gene to demonstrate that the bacilysin negative phenotype is actually due to the mutation in the yvfI gene and not a polar effect of yvfI mutation on the downstream gene. As expected, all resulting lacR mutant derivatives of PY79 still produced bacilysin.

bac genes for recombinant bacilysin and anticapsin production in Bacillus host strains

The genes encoding the biosynthesis of the dipeptide bacilysin and its antibiotic constituent anticapsin were isolated from several strains of Bacillus subtilis as well as B. amyloliquefaciens and B. pumilus. The ywfBCDEF genes of B. subtilis 168 were shown to carry the biosynthetic core functions and were renamed bacABCDE. Mutation of the bacD gene or transformation of the bacABC genes into a B. subtilis Delta (ywfA-bacABCDE) deletion mutant led to the accumulation of anticapsin, which was fourfold higher after transformation of the bacABC genes into a bacD mutant. The genes bacD and bacE proved to encode the functions of amino acid ligation and self-protection to bacilysin, respectively. Amplification of the bacABCDE gene cluster in a bacAB gene-deficient host strain of B. amyloliquefaciens resulted in a tenfold bacilysin overproduction. Some host strains required distinct glucosamine and yeast extract supplements in order to prevent suicidal effects of the recombinant antibiotic production. The bac genes from different Bacillus species revealed the same arrangement and 72.6-88.6% of sequence identity.

Tn10 insertional mutations of Bacillus subtilis that block the biosynthesis of bacilysin

Transposon mutagenesis was employed to isolate the gene(s) related with the biosynthesis of dipeptide antibiotic in Bacillus subtilis PY79 (a prototrophic derivative of the standard 168 strain). The blocked mutants were phenotypically selected from the transposon library by bioassay and the complete loss of biosynthetic ability was verified through ESI-mass spectrometry analysis. Four different bacilysin nonproducer mutants (Bac(-)::Tn10(ori-spc)) were isolated from the transposon library. The genes involved in bacilysin biosynthesis were identified as thyA (thymidilate synthetase), ybgG (unknown; similar to homocysteine methyl transferase) and oppA (oligopeptide permease), respectively. The other blocked gene was yvgW (unknown; similar to heavy metal-transporting ATPase); however, backcross studies did not verify its involvement in bacilysin biosynthesis. This gene, on the other hand, appeared to be necessary for efficient sporulation and transformation. Opp involvement was significant as it suggested that bacilysin biosynthesis is under or a component of the quorum sensing pathway which has been shown to be responsible for the establishment of sporulation, competence development and onset of surfactin biosynthesis. For verification, it was necessary to check the involvement of peptide pheromones (PhrA or PhrC) internalized by the Opp system and response regulator ComA as the essential components of this global control. phrA, phrC and comA deleted mutants of PY79 were thus constructed and the latter two genes were shown to be essential for bacilysin biosynthesis.

The natural functions of secondary metabolites

Secondary metabolites, including antibiotics, are produced in nature and serve survival functions for the organisms producing them. The antibiotics are a heterogeneous group, the functions of some being related to and others being unrelated to their antimicrobial activities. Secondary metabolites serve: (i) as competitive weapons used against other bacteria, fungi, amoebae, plants, insects, and large animals; (ii) as metal transporting agents; (iii) as agents of symbiosis between microbes and plants, nematodes, insects, and higher animals; (iv) as sexual hormones; and (v) as differentiation effectors. Although antibiotics are not obligatory for sporulation, some secondary metabolites (including antibiotics) stimulate spore formation and inhibit or stimulate germination. Formation of secondary metabolites and spores are regulated by similar factors. This similarity could insure secondary metabolite production during sporulation. Thus the secondary metabolite can: (i) slow down germination of spores until a less competitive environment and more favorable conditions for growth exist; (ii) protect the dormant or initiated spore from consumption by amoebae; or (iii) cleanse the immediate environment of competing microorganisms during germination.

Biotechnological developments in Turkey

Turkey is a country not usually associated with industrial biotechnology. However, when current research potential in universities and other R & D centers and particularly contributions made to the international literature since the mid-1980s are taken into account, high-quality international-level work is now commonplace, especially in areas such as industrial microbiology, enzyme technology, biomaterials and biological wastewater treatment. Work in plant biotechnology is at a relatively early stage, but is expected to become a rapidly developing force in the near future. The present article documents current potential in Turkey, based on significant publications produced during the last 8 years.