Piceamycin and its N-acetylcysteine adduct is produced by Streptomyces sp. GB 4-2.

Piceamycin, a new macrolactam polyketide antibiotic, was detected by HPLC-diode array screening in extracts of Streptomyces sp. GB 4-2, which was isolated from the mycorrhizosphere of Norway spruce. The structure of piceamycin was determined by mass spectrometry and NMR experiments. It showed inhibitory activity against Gram-positive bacteria, selected human tumor cell lines and protein tyrosine phosphatase 1B.

An acyl-CoA dehydrogenase is involved in the formation of the Delta cis3 double bond in the acyl residue of the lipopeptide antibiotic friulimicin in Actinoplanes friuliensis.

The lipopeptide antibiotic friulimicin, produced by Actinoplanes friuliensis, is an effective drug against Gram-positive bacteria, such as methicillin-resistant Staphylococcus epidermidis and Staphylococcus aureus strains. Friulimicin consists of a cyclic peptide core of ten amino acids and an acyl residue linked to an exocyclic amino acid. The acyl residue is essential for antibiotic activity, varies in length from C13 to C15, and carries a characteristic double bond at position Delta cis3. Sequencing of a DNA fragment adjacent to a previously described fragment encoding some of the friulimicin biosynthetic genes revealed several genes whose gene products resemble enzymes of lipid metabolism. One of these genes, lipB, encodes an acyl-CoA dehydrogenase homologue. To elucidate the function of the LipB protein, a lipB insertion mutant was generated and the friulimicin derivative (FR242) produced by the mutant was purified. FR242 had antibiotic activity lower than friulimicin in a bioassay. Gas chromatography showed that the acyl residue of wild-type friulimicin contains a double bond, whereas a saturated bond was present in FR242. These results were confirmed by the heterologous expression of lipB in Streptomyces lividans T7, which led to the production of unsaturated fatty acids not found in the S. lividans T7 parent strain. These results indicate that the acyl-CoA dehydrogenase LipB is involved in the introduction of the unusual Delta cis3 double bond into the acyl residue of friulimicin.

A unique hydrophobic cluster near the active site contributes to differences in borrelidin inhibition among threonyl-tRNA synthetases.

Borrelidin, a compound with anti-microbial and anti-angiogenic properties, is a known inhibitor of bacterial and eukaryal threonyl-tRNA synthetase (ThrRS). The inhibition mechanism of borrelidin is not well understood. Archaea contain archaeal and bacterial genre ThrRS enzymes that can be distinguished by their sequence. We explored species-specific borrelidin inhibition of ThrRSs. The activity of ThrRS from Sulfolobus solfataricus and Halobacterium sp. NRC-1 was inhibited by borrelidin, whereas ThrRS enzymes from Methanocaldococcus jannaschii and Archaeoglobus fulgidus were not. In Escherichia coli ThrRS, borrelidin binding induced a conformational change, and threonine was not activated as shown by ATP-PP(i) exchange and a transient kinetic assay measuring intrinsic tryptophan fluorescence changes. These assays further showed that borrelidin is a noncompetitive tight binding inhibitor of E. coli ThrRS with respect to threonine and ATP. Genetic selection of borrelidin-resistant mutants showed that borrelidin binds to a hydrophobic region (Thr-307, His-309, Cys-334, Pro-335, Leu-489, Leu-493) proximal to the zinc ion at the active site of the E. coli ThrRS. Mutating residue Leu-489 --> Trp reduced the space of the hydrophobic cluster and resulted in a 1500-fold increase of the K(i) value from 4 nM to 6 microm. An alignment of ThrRS sequences showed that this cluster is conserved in most organisms except for some Archaea (e.g. M. jannaschii, A. fulgidus) and some pathogens (e.g. Helicobacter pylori). This study illustrates how one class of natural product inhibitors affects aminoacyl-tRNA synthetase function, providing potentially useful information for structure-based inhibitor design.

Profound insights into squalene cyclization.

In this issue of Chemistry & Biology, our understanding of the formation of pentacyclic hopene from the linear squalene is enhanced by an X-ray structure of a complex between squalene-hopene cyclase and the substrate analog 2-azasqualene.

Occurrence of Cationic Intermediates and Deficient Control during the Enzymatic Cyclization of Squalene to Hopanoids.

Tetracyclic triterpenes (for example, 3) are formed as minor products of the cyclization of squalene (1) by squalene/hopene cyclase; they have also been found in vivo. The identification of these side products offers new insight into the complex biosynthesis of the hopane skeleton, and proves that the enzyme has no absolute control of the conformation of the acyclic substrate squalene and that tetra- and pentacyclic carbocationic intermediates are involved (for example, 2) in the cyclization process.

Squalene-hopene cyclase from Bradyrhizobium japonicum: cloning, expression, sequence analysis and comparison to other triterpenoid cyclases.

With the help of a PCR-based screening method, the gene encoding squalenehopene cyclase (SHC) of Bradyrhizobium japonicum USDA 110 was isolated from a cosmid library. The SHC catalyses the cyclization of squalene to hopanoids, a class of triterpenoid lipids recently discovered in nitrogen-fixing, root-nodule-forming Bradyrhizobium bacteria. Hybridization experiments showed that the gene is present in bacteria of all Bradyrhizobium strains tested and in photosynthetic bacteria forming stem nodules on tropical legumes of the genus Aeschynomene. The Bradyrhizobium shc gene is 1983 bp in length and encodes a protein of 660 amino acid residues with a calculated molecular mass of 73671 Da. Comparison of the deduced amino acid sequence with the sequences of other SHCs revealed highest similarity (70%) to the SHC from the Gram-negative Zymomonas mobilis and lower similarity (48%) to the SHCs from the Gram-positive Alicyclobacillus acidocaldarius and Alicyclobacillus acidoterrestris. Bradyrhizobium SHC also showed similarity (38-43%) to eukaryotic oxidosqualene cyclases. The B. japonicum shc gene was expressed in Escherichia coli. The recombinant SHC catalysed the cyclization of squalene to the hopanoids hopene and diplopterol in vitro. However, the formation of the gammacerane derivative tetrahymanol, which is produced in addition to hopanoids in B. japonicum strains in vivo, could not be detected in vitro. Therefore, the presence of a second squalene cyclase in B. japonicum can be assumed. Sequence analysis of 0.5 kb upstream from the shc gene identified a partial ORF with significant similarity to the C-terminus of an ORF located immediately upstream from the shc gene in Z. mobilis. Both ORFs also showed similarity to phytoene desaturases from cyanobacteria and plants. The 3'-end of this ORF from B. japonicum overlaps with 13 bp at the 5'-end of shc. The close proximity of this ORF to shc suggests that shc and this ORF may be part of an operon.

Biosynthetic Studies of omega-Cycloheptyl Fatty Acids in Alicyclobacillus cycloheptanicus. Formation of Cycloheptanecarboxylic Acid from Phenylacetic Acid.

The formation of the structurally novel, mono-substituted cycloheptane ring in omega-cycloheptyl fatty acids in Alicyclobacillus cycloheptanicus (formerly Bacillus cycloheptanicus) has been examined. Feeding experiments with (13)C- and (2)H-labeled intermediates demonstrated that cycloheptanecarboxylic acid (3), probably as its CoA thioester, is the starter unit for omega-cycloheptyl fatty acid biosynthesis. Analysis of the resultant labeling pattern from a feeding experiment with [U-(13)C(6)]glucose suggested a shikimate pathway origin of 3 via aromatic amino acids. [1,2-(13)C(2)]Phenylacetic acid (6) was efficiently metabolized into the 3-derived moiety in a manner reminiscent of the seven-membered ring Pseudomonas metabolite thiotropocin. The fates of the aromatic and benzylic hydrogens of 6 were determined; these dictated various boundary conditions for the biosynthetic pathway from 6 to 3. Taken together with the results from feeding experiments with postulated cycloheptenylcarboxylate biosynthetic intermediates, the data lead us to propose a pathway which involves an oxidative ring-expansion of 6 to a hydroxynorcaradiene intermediate followed by a series of double bond reductions and dehydrations to the saturated 3.