The Gram-negative phytopathogen Xanthomonas campestris pv. campestris employs a 5'UTR as a feedback controller to regulate methionine biosynthesis.

The synthesis of methionine is critical for most bacteria. It is known that cellular methionine has a feedback effect on the expression of met genes involved in de novo methionine biosynthesis. Previous studies revealed that Gram-negative bacteria control met gene expression at the transcriptional level by regulator proteins, while most Gram-positive bacteria regulate met genes at post-transcriptional level by RNA regulators (riboregulators) located in the 5'UTR of met genes. However, despite its importance, the methionine biosynthesis pathway in the Gram-negative Xanthomonas genus that includes many important plant pathogens is completely uncharacterized. Here, we address this issue using the crucifer black rot pathogen Xanthomonas campestris pv. campestris (Xcc), a model bacterium in microbe-plant interaction studies. The work identified an operon (met) involved in de novo methionine biosynthesis in Xcc. Disruption of the operon resulted in defective growth in methionine-limited media and in planta. Western blot analysis revealed that the expression of the operon is dependent on methionine levels. Further molecular analyses demonstrated that the 5'UTR, but not the promoter of the operon, is involved in feedback regulation on operon expression in response to methionine availability, providing an example of a Gram-negative bacterium utilizing a 5'UTR region to control the expression of the genes involved in methionine biosynthesis.

A SAM-I riboswitch with the ability to sense and respond to uncharged initiator tRNA.

All known riboswitches use their aptamer to senese one metabolite signal and their expression platform to regulate gene expression. Here, we characterize a SAM-I riboswitch (SAM-IXcc) from the Xanthomonas campestris that regulates methionine synthesis via the met operon. In vitro and in vivo experiments show that SAM-IXcc controls the met operon primarily at the translational level in response to cellular S-adenosylmethionine (SAM) levels. Biochemical and genetic data demonstrate that SAM-IXcc expression platform not only can repress gene expression in response to SAM binding to SAM-IXcc aptamer but also can sense and bind uncharged initiator Met tRNA, resulting in the sequestering of the anti-Shine-Dalgarno (SD) sequence and freeing the SD for translation initiation. These findings identify a SAM-I riboswitch with a dual functioning expression platform that regulates methionine synthesis through a previously unrecognized mechanism and discover a natural tRNA-sensing RNA element. This SAM-I riboswitch appears to be highly conserved in Xanthomonas species.

WITHDRAWN: Transplantation of bone mesenchymal stem cells containing the nerve growth factor gene in adult rat brain with Fimbria Fornix lesion.

Bone marrow-derived mesenchymal stem cells (BMSCs) are a promising source for cell-based treatment of brain injury, but the therapy of BMSCs is restricted by low cell survival. We examined whether nerve growth factor (NGF) improve BMSCs viability in the brain with Fimbria-Fornix lesion (FF). After transduction of NGF gene via recombinant retroviral vectors, the rat BMSCs were transformed into the NGF-GFP positive BMSCs, nearly 100% of cells expressed NGF. After transplanted into basal forebrain of rat with FF, the NGF-GFP positive BMSCs expressed the exogenous NGF gene in the host brain, and interesting, the survival number of BMSCs in the NGF group was significant more than that of the void plasmid group. Furthermore, the number of choline acetyltransferase (ChAT) immunoreactive neurons of NGF group was also significant higher than those of the void plasmid group (p<0.05) or the PBS group (p<0.01). Performance in the water maze test was improved in these rats in NGF group. These results indicate that NGF increased BMSCs survival in brain with FF, which results in better improvement of brain function than injected with BMSCs alone.