CHANGES IN THE COMPOSITION OF AN ACTINOMYCIN COMPLEX DURING GROWTH OF A STREPTOMYCES CULTURE

Genetic interrelations in the actinomycin biosynthetic gene clusters of Streptomyces antibioticus IMRU 3720 and Streptomyces chrysomallus ATCC11523, producers of actinomycin X and actinomycin C

Sequencing the actinomycin (acm) biosynthetic gene cluster of Streptomyces antibioticus IMRU 3720, which produces actinomycin X (Acm X), revealed 20 genes organized into a highly similar framework as in the bi-armed acm C biosynthetic gene cluster of Streptomyces chrysomallus but without an attached additional extra arm of orthologues as in the latter. Curiously, the extra arm of the S. chrysomallus gene cluster turned out to perfectly match the single arm of the S. antibioticus gene cluster in the same order of orthologues including the the presence of two pseudogenes, scacmM and scacmN, encoding a cytochrome P450 and its ferredoxin, respectively. Orthologues of the latter genes were both missing in the principal arm of the S. chrysomallus acm C gene cluster. All orthologues of the extra arm showed a G +C-contents different from that of their counterparts in the principal arm. Moreover, the similarities of translation products from the extra arm were all higher to the corresponding translation products of orthologue genes from the S. antibioticus acm X gene cluster than to those encoded by the principal arm of their own gene cluster. This suggests that the duplicated structure of the S. chrysomallus acm C biosynthetic gene cluster evolved from previous fusion between two one-armed acm gene clusters each from a different genetic background. However, while scacmM and scacmN in the extra arm of the S. chrysomallus acm C gene cluster are mutated and therefore are non-functional, their orthologues saacmM and saacmN in the S. antibioticus acm C gene cluster show no defects seemingly encoding active enzymes with functions specific for Acm X biosynthesis. Both acm biosynthetic gene clusters lack a kynurenine-3-monooxygenase gene necessary for biosynthesis of 3-hydroxy-4-methylanthranilic acid, the building block of the Acm chromophore, which suggests participation of a genome-encoded relevant monooxygenase during Acm biosynthesis in both S. chrysomallus and S. antibioticus.

DNA as an adhesin: Bacillus cereus requires extracellular DNA to form biofilms

The soil saprophyte Bacillus cereus forms biofilms at solid-liquid interfaces. The composition of the extracellular polymeric matrix is not known, but biofilms of other bacteria are encased in polysaccharides, protein, and also extracellular DNA (eDNA). A Tn917 screen for strains impaired in biofilm formation at a solid-liquid interface yielded several mutants. Three mutants deficient in the purine biosynthesis genes purA, purC, and purL were biofilm impaired, but they grew planktonically like the wild type in Luria-Bertani broth. Biofilm populations had higher purA, purC, and purL transcript ratios than planktonic cultures, as measured by real-time PCR. Laser scanning confocal microscopy (LSCM) of BacLight-stained samples indicated that there were nucleic acids in the cell-associated matrix. This eDNA could be mobilized off the biofilm into an agarose gel matrix through electrophoresis, and it was a substrate for DNase. Glass surfaces exposed to exponentially growing populations acquired a DNA-containing conditioning film, as indicated by LSCM. Planktonic exponential-phase cells released DNA into an agarose gel matrix through electrophoresis, while stationary-phase populations did not do this. DNase treatment of planktonic exponential-phase populations rendered cells more susceptible than control populations to the DNA-interacting antibiotic actinomycin D. Exponential-phase purA cells did not contain detectable eDNA, nor did they convey a DNA-containing conditioning film to the glass surface. These results indicate that exponential-phase cells of B. cereus ATCC 14579 are decorated with eDNA and that biofilm formation requires DNA as part of the extracellular polymeric matrix.

Effect of pipecolic acid isomers on the biogenesis of actinomycins

Streptomyces antibioticus produces a family of actinomycin components which differ solely at the "imino acid" site of the molecule; however, new congeners of actinomycin are synthesized when cultures are supplemented with pipecolic acid (PA). The quantitative and qualitative nature of the actinomycins produced differed when cultures were incubated in the presence of either l-, d-, or racemic PA. In the presence of d-PA, the quantitative and qualitative nature of those actinomycins produced were similar to those produced in the absence of supplementation. By contrast, in the presence of l-PA or dl-PA new actinomycin components containing PA were synthesized at the expense of normally produced components. Also, the total amount of antibiotic produced decreased in response to increasing concentrations of exogenously supplied l-PA. This effect occurred whether or not d-PA was also added to cultures. Concentrations of l-PA greater than 125 mug/ml of medium resulted in no additional decrease in the amount of antibiotic produced. Although PA-containing actinomycins were formed as early as 6 h after the addition of 250 mug of l-PA per ml, it was not until 24 h postaddition that the relative percentages of actinomycin components produced approached a constant value. Evidence presented here indicated that the l-isomer of PA replaces l-proline in the pentapeptide of actinomycin.

Influence of isoleucine upon quinomycin biosynthesis by Streptomyces sp. 732

When dl-isoleucine was added to an ammonium nitrate-maltose medium during cultivation of Streptomyces sp. 732, quinomycin B formation was selectively enhanced (from 3 to 70% of the quinomycin mixture) and quinomycin C synthesis was inhibited completely. In addition, two new quinomycins, designated quinomycins D and E, were produced in the presence of isoleucine. These compounds were found to contain N-methylalloisoleucine. In experiments with isoleucine enantiomorphs, it was determined that the order of effectiveness for quinomycin B synthesis was dl-isoleucine > d-isoleucine > l-isoleucine. The extent to which quinomycin B synthesis is enhanced depends upon the concentration and the time of addition of isoleucine to the medium. The effect of dl-isoleucine was reduced to some extent by the addition of l-valine. It is conceivable that amino acids which are precursors of the N-methylamino acids in quinomycin can regulate quinomycin formation.