sanC--a novel gene involved in nikkomycin biosynthesis in Streptomyces ansochromogenes

Cryptic phosphorylation in nucleoside natural product biosynthesis

Kinases are annotated in many nucleoside biosynthetic gene clusters (BGCs) but generally are considered responsible only for self-resistance. Here, we report an unexpected 2’-phosphorylation of nucleoside biosynthetic intermediates in the nikkomycin and polyoxin pathways. This phosphorylation is a unique cryptic modification as it is introduced in the third of seven steps during aminohexuronic acid (AHA) nucleoside biosynthesis, retained throughout the pathway’s duration, and is removed in the last step of the pathway. Bioinformatic analysis of reported nucleoside BGCs suggests the presence of cryptic phosphorylation in other pathways and the importance of functional characterization of kinases in nucleoside biosynthetic pathways in general. This study also functionally characterized all of the enzymes responsible for AHA biosynthesis and revealed that AHA is constructed via a unique oxidative C-C bond cleavage reaction. The results suggest a divergent biosynthetic mechanism for three classes of antifungal nucleoside natural products.

Identification and Characterization of sanH and sanI Involved in the Hydroxylation of Pyridyl Residue During Nikkomycin Biosynthesis in Streptomyces ansochromogenes

Nikkomycins are highly potent inhibitors of chitin synthase. The nikkomycin biosynthetic gene cluster has been cloned from Streptomyces asochromogenes. Two cytochrome P450 monooxygenase genes (sanQ, sanH) and one ferredoxin gene (sanI) were found in the cluster. It was reported that SanQ is involved in the hydroxylation of L-His, a key step in 4-formyl-4-imidazolin-2-one base biosynthesis. Here, we have studied the function of sanH and sanI. Disruption of sanH abolished the production of nikkomycin X and Z, but it accumulated one dominant component nikkomycin Lx, which is the nikkomycin X analog lacking the hydroxy group at the pyridyl residue. The sanI disruption mutant accumulated predominantly nikkomycin Lx in addition to nikkomycin X and Z. The nikkomycin production profile of the sanH and sanI double disruption mutant was the same as that of the sanH disruption mutant. These results confirmed that SanH is essential for the hydroxylation of pyridyl residue in nikkomycin biosynthesis of S. ansochromogenes and first demonstrated that SanI is an effective electron donor for SanH, but not for SanQ in vivo.

Antibiotic biosynthetic pathways and pathway engineering--a growing research field in China

This review describes the recent research activities in China in relation to studies on antibiotic biosynthetic pathways and pathway engineering in actinomycetes. 75 references are cited.

SanJ, an ATP-dependent picolinate-CoA ligase, catalyzes the conversion of picolinate to picolinate-CoA during nikkomycin biosynthesis in Streptomyces ansochromogenes

Nikkomycins, a group of peptidyl nucleoside antibiotics, are competitive inhibitors of chitin synthase. The nikkomycin biosynthetic gene cluster has been cloned previously from Streptomyces ansochromogenes. The cluster contains 25 complete ORFs including sanJ. The sanJ gene was inactivated by the insertion of a kanamycin resistance gene and the resulting disruption mutants failed to produce nikkomycins. Moreover, the nikkomycin production was restored by complementation with a single copy of sanJ. The deduced product of sanJ bears striking sequence similarity with enzymes belonging to the adenylate-forming superfamily. sanJ was overexpressed as a His6-tagged fusion protein in Escherichia coli and purified to apparent homogeneity by affinity chromatography. The purified SanJ demonstrated adenylate ligase activity in the presence of picolinate or its analogs (benzoate, nicotinate, 4-methoxybenzoate, 4-hydroxybenzoate), ATP and Mg2+. SanJ was also found to catalyze the conversion of picolinate, benzoate, nicotinate to their corresponding CoA esters and 4-methoxybenzoate, 4-hydroxybenzoate to their respective AMP derivatives in vitro. This was unambiguously shown by using HPLC and electrospray ionization mass spectrometry (ESI-MS) or by comparing the reaction product with an authentic standard of benzoyl-CoA. These results indicated that sanJ encodes an ATP-dependent picolinate-CoA ligase which is essential for nikkomycin biosynthesis.

Improvement of nikkomycin production by enhanced copy of sanU and sanV in Streptomyces ansochromogenes and characterization of a novel glutamate mutase encoded by sanU and sanV

Previous studies revealed that two genes-sanU and sanV were associated with nikkomycin biosynthesis in Streptomyces ansochromogenes. A plasmid used to increase an extra copy of sanU and sanV was constructed and introduced into wild-type strain. HPLC results showed that nikkomycin production of recombinant strain was about 1.8 fold than that of wild-type strain. RT-PCR analysis indicated that the transcriptional level of sanU and sanV in this recombinant strain was about two folds than that of wild-type strain. The sanU and sanV were expressed in E. coli BL21 (DE3). SanU and SanV were purified individually. SanU and SanV assembled with coenzyme B12 to form a complete enzyme in vitro, which showed glutamate mutase activity. The glutamate mutase converted L-glutamate toL-threo-beta-Methylaspartic acid, and then l-threo-beta-Methylaspartic acid was probably deaminated to form 2-oxo-3-methylsuccinic acid to join biosynthetic pathway of the peptidyl moiety HPHT in S. ansochromogenes. SanU is the coenzyme B12-binding component and more than two folds of SanU are required for maximal enzyme activity. The optimal pH and temperature for the formed enzyme are 7.5-8.5 and 35-42 degrees C, respectively. Sulfhydryl compounds are important for activity of the reassembled enzyme.

Cloning and characterization of sanO, a gene involved in nikkomycin biosynthesis in Streptomyces ansochromogenes

AIMS

To clone and characterize sanO, a gene involved in the biosynthesis of nikkomycins in Streptomyces ansochromogenes.

METHODS AND RESULTS

A 4.5-kb BamHI-KpnI fragment was cloned and sequenced. Sequence analysis revealed that this fragment contains three complete open reading frames. The largest one with 2034 bp was designated sanO, which encodes a protein consisting of 667 amino acids with high similarity to module of peptide synthetase. sanO disruption mutants produced no nikkomycin X, but formed nikkomycin Z at almost the same level of the wild-type strain. The production of nikkomycin X can be recovered by genetic complementation of sanO disruption mutants. Primer extension also revealed that transcription start point(tsp) of sanO was localized 87 bp upstream of the potential start codon (GTG).

CONCLUSIONS

sanO was essential for the biosynthesis of nikkomycin X.

SIGNIFICANCE AND IMPACT OF THE STUDY

Nikkomycins have received increased interest of study because of their prospective application in agriculture and medicine. Cloning and characterization of genes involved in the nikkomycin biosynthesis will help to elucidate the whole biosynthetic pathway of nikkomycins.