Properties and localisation of mycobacillin-synthesising enzyme system in Bacillus subtilis B3

Antimicrobial Bacillus: Metabolites and Their Mode of Action

The agricultural industry utilizes antibiotic growth promoters to promote livestock growth and health. However, the World Health Organization has raised concerns over the ongoing spread of antibiotic resistance transmission in the populace, leading to its subsequent ban in several countries, especially in the European Union. These restrictions have translated into an increase in pathogenic outbreaks in the agricultural industry, highlighting the need for an economically viable, non-toxic, and renewable alternative to antibiotics in livestock. Probiotics inhibit pathogen growth, promote a beneficial microbiota, regulate the immune response of its host, enhance feed conversion to nutrients, and form biofilms that block further infection. Commonly used lactic acid bacteria probiotics are vulnerable to the harsh conditions of the upper gastrointestinal system, leading to novel research using spore-forming bacteria from the genus Bacillus. However, the exact mechanisms behind Bacillus probiotics remain unexplored. This review tackles this issue, by reporting antimicrobial compounds produced from Bacillus strains, their proposed mechanisms of action, and any gaps in the mechanism studies of these compounds. Lastly, this paper explores omics approaches to clarify the mechanisms behind Bacillus probiotics.

Characterization of three-fraction mycobacillin synthetase

Mycobacillin synthetase lacks aspartic acid racemase, alanine racemase and glutamic acid racemase activities. The enzyme also does not respond to ATP-[32P]Pi exchange, nor does it catalyse the antibiotic synthesis in presence of amino acids of configuration opposite to that present in the molecule. Preincubation with optical isomers of opposite configuration inhibited the ATP-[32P]Pi exchange reaction to the extent of 60-90%. None of the three fractions of mycobacillin synthetase contained a pantothenic acid arm. Two molecules of ATP are required to synthesize one peptide bond of mycobacillin. Intermediate peptides of mycobacillin are not covalently linked to the three-fraction mycobacillin synthetase.

Purification of the constituent enzyme fractions of mycobacillin synthetase

The final purification of the three-fraction enzyme complex mycobacillin synthetase was done by hydroxyapatite column chromatography and sucrose-density-gradient centrifugation; each of the fractions obtained migrates as a single component in SDS/polyacrylamide-gel electrophoresis and gel electrofocusing. The Mr of the enzyme fractions A, B and C by gel filtration is 260 000, 190 000 and 105 000, and that by SDS/polyacrylamide-gel electrophoresis is 252 000, 198 000 and 108 000 respectively. None of the enzyme fractions appears to possess subunit structure.

Functional characterization of constituent enzyme fractions of mycobacillin synthetase

The enzyme fraction A, a constituent enzyme of the three-fraction enzyme mycobacillin synthetase, independently and sequentially activated five amino acids starting from L-proline, producing the pentapeptide Pro(Asp1,Glu1,Tyr1)Asp. The fractions B and C were unable to function independently. However, the fraction B synthesized the nonapeptide Pro(Asp3,Glu1,Tyr2,Ser1)Leu, sequentially activating the pentapeptide and next four amino acids, whereas the fraction C synthesized mycobacillin by the sequential activation of the nonapeptide and the remaining four amino acids. The pH optima of the above enzymes are almost identical (pH 7.8), but their Km values are a little different.

Translocation of mycobacillin synthetase in Bacillus subtilis

The extracellular release of mycobacillin from Bacillus subtilis first occurred in the medium at the onset of stationary phase and continued at a high rate even after 6 days. Mycobacillin synthetase activity appeared earlier than late-exponential phase in the cytosol of producer cells and was not sedimentable even at 105 000 g. The activity then quickly reached the maximum late in the stationary phase. With further increase in the age of the culture, the activity gradually disappeared from the cytosol, to reappear concomitantly in the membrane in an insoluble particulate form, even in absence of protein synthesis. The membrane-bound synthetase activity was sedimentable at 10 000 g and was fairly active even after 5 days.

Standardization of parameters for the mycobacillin synthetase activity

An effective method of preparation involving sonication was developed for cell-free mycobacillin synthetase from Bacillus subtilis. The enzyme showed optimum activity at a buffer concentration of 50 mM (Tris-HCl) and pH 7.5. ATP and Mg2+ which were essential for synthesis showed an optimum requirement at a ratio of 1:1. The synthetase was markedly inhibited by ADP whereas AMP was without any effect. ATP or ATP-generating system could not be replaced by GTP, UTP or CTP. Co2+ and Mn2+ could to some extent substitute Mg2+. Mercapto reagents inhibited the antibiotic synthesis. Exogenous addition of pantothenic acid had no effect.

Fractionation of the mycobacillin-synthesizing enzyme system

The mycobacillin-synthesizing enzyme system was highly purified by fractionation at 30-55% (NH4)2SO4 saturation. The enzyme concentrate on Sephadex G-200 gel chromatography was resolved into three distinct fragments. Each of the fragments on further purification by DEAE-cellulose ion-exchange chromatography behaved as a single-component system, as clearly indicated by the sharpness of the peaks in the elution diagram. None of the fragments alone nor any two of them in all possible combinations possessed mycobacillin-synthesizing activity, which was restored only when the three fragments were used together in the test system.

Peptides from a mycobacillin-synthesizing cell-free system

In a cell-free system from Bacillus subtilis B(3), ATP-P(i) exchange was catalysed by l-proline at a pH optimum of 7.2. Further stimulation by component amino acids of mycobacillin was inhibited by deprivation from the synthesizing system of even a single amino acid occurring at any point of the cyclic peptide. This inhibition, however, decreased with the distance in the molecule of the given amino acid from l-proline. Peptides containing respectively two, three, four, five and six amino acids were isolated from the mycobacillin-synthesizing system by an amino acid-deprivation technique. The amino acid composition of these peptides and also their N- and C-terminal amino acid residues were the same as those of peptides that would be obtained if mycobacillin synthesis occurred starting from l-proline and was interrupted at various points along the polypeptide chain.