The gyrase inhibitor albicidin consists of p-aminobenzoic acids and cyanoalanine

Albicidin is a potent DNA gyrase inhibitor produced by the sugarcane pathogenic bacterium Xanthomonas albilineans. Here we report the elucidation of the hitherto unknown structure of albicidin, revealing a unique polyaromatic oligopeptide mainly composed of p-aminobenzoic acids. In vitro studies provide further insights into the biosynthetic machinery of albicidin. These findings will enable structural investigations on the inhibition mechanism of albicidin and its assessment as a highly effective antibacterial drug.

First Report of Acidovorax avenae subsp. avenae Causing Sugarcane Red Stripe in Gabon

During a disease inspection at the sugarcane estate SUCAF near Franceville, Gabon, in March 2011, 1- to 3-mm wide and several dm long dark red stripes were observed on sugarcane (Saccharum spp.) leaves of many plants of cultivar R570. These plants were 5.5 months old in the first ratoon crop. Additionally, spindle leaves of several stalks were rotted and could be easily pulled out of the top of the stalk. Longitudinal sections of diseased stalks showed reddish-brown discoloration of the upper stem and the rotted spindle had an unpleasant odor. Circular, convex, smooth, yellow-cream pigmented bacterial colonies with 2 to 3 mm diameter were isolated after 3 days at 28°C from young leaf lesions on YPGA (yeast extract 7 g/L, peptone 7 g/L, glucose 7 g/L, agar 15 g/L, pH 6.8 to 7.0). The 16S-23S internal transcribed spacer (ITS) of two representative colonies was PCR amplified, and the nucleotide sequences were shown to be 99% identical to the 16S-23S ITS sequence from the genome of Acidovorax avenae subsp. avenae strain ATCC 19860 (GenBank: CP002521.1). One of these A. avenae subsp. avenae isolates from Gabon was inoculated into greenhouse grown plants of sugarcane cultivar R570. Plants were inoculated by injection into the sheath of spindle leaves above the meristem with the bacterial strain (12 plants) or with a water control (six plants). In this method, the bacteria (10⁸ CFU/ml) were injected using a syringe through the leaf sheath until filling the leaf whorl. Three weeks post-inoculation, one to several cm long red-brown stripes were observed on leaves of 11 of 12 inoculated plants. Seven weeks post-inoculation, all plants exhibited symptoms, from red, brown, or black stripes to leaf necrosis, rotting, and death of the spindle leaves (six plants). All six control plants were symptomless. In a second experiment, 6 of 12 plants showed symptoms 3 weeks post inoculation, and the pathogen was successfully re-isolated from all six symptomatic plants with YPGA medium. The 16S-23S ITS of three single colonies obtained each from different symptomatic plants were PCR amplified and the nucleotide sequences were again found 99% identical to the 16S-23S ITS sequence from the genome of A. avenae subsp. avenae ATCC 19860. To our knowledge, this is the first report of A. avenae subsp. avenae, the causal agent of sugarcane red stripe (also reported as top rot), in Gabon. It is also the first description of the occurrence of the top rot form of the disease in R570, a cultivar that is grown in several locations of Africa, the Mascarene Islands, and the French West Indies. A large-scale survey needs to be undertaken to determine the distribution of red stripe in Gabon, a disease for which several outbreaks have been reported recently worldwide (1,2). References: (1) M. P. Grisham and R. M. Johnson. Phytopathology 101:564, 2011. (2) S. Zia-ul-Hussnain et al. Afr. J. Biotechnol. 10:7191, 2011.

Genome mining reveals the genus Xanthomonas to be a promising reservoir for new bioactive non-ribosomally synthesized peptides

BACKGROUND

Various bacteria can use non-ribosomal peptide synthesis (NRPS) to produce peptides or other small molecules. Conserved features within the NRPS machinery allow the type, and sometimes even the structure, of the synthesized polypeptide to be predicted. Thus, bacterial genome mining via in silico analyses of NRPS genes offers an attractive opportunity to uncover new bioactive non-ribosomally synthesized peptides. Xanthomonas is a large genus of Gram-negative bacteria that cause disease in hundreds of plant species. To date, the only known small molecule synthesized by NRPS in this genus is albicidin produced by Xanthomonas albilineans. This study aims to estimate the biosynthetic potential of Xanthomonas spp. by in silico analyses of NRPS genes with unknown function recently identified in the sequenced genomes of X. albilineans and related species of Xanthomonas.

RESULTS

We performed in silico analyses of NRPS genes present in all published genome sequences of Xanthomonas spp., as well as in unpublished draft genome sequences of Xanthomonas oryzae pv. oryzae strain BAI3 and Xanthomonas spp. strain XaS3. These two latter strains, together with X. albilineans strain GPE PC73 and X. oryzae pv. oryzae strains X8-1A and X11-5A, possess novel NRPS gene clusters and share related NRPS-associated genes such as those required for the biosynthesis of non-proteinogenic amino acids or the secretion of peptides. In silico prediction of peptide structures according to NRPS architecture suggests eight different peptides, each specific to its producing strain. Interestingly, these eight peptides cannot be assigned to any known gene cluster or related to known compounds from natural product databases. PCR screening of a collection of 94 plant pathogenic bacteria indicates that these novel NRPS gene clusters are specific to the genus Xanthomonas and are also present in Xanthomonas translucens and X. oryzae pv. oryzicola. Further genome mining revealed other novel NRPS genes specific to X. oryzae pv. oryzicola or Xanthomonas sacchari.

CONCLUSIONS

This study revealed the significant potential of the genus Xanthomonas to produce new non-ribosomally synthesized peptides. Interestingly, this biosynthetic potential seems to be specific to strains of Xanthomonas associated with monocotyledonous plants, suggesting a putative involvement of non-ribosomally synthesized peptides in plant-bacteria interactions.