THREE DIMENSIONAL STRUCT: IMPERATOXIN A, PEPTIDE LIGAND OF CA2+ RELEASE CHANNELS

三叶草毒素（Trifolitoxin，TFX）是由根瘤菌产生的一种肽类细菌素 豆科植物 三叶草T24，对大多数 菌株R.豆科和豆科R.弗雷迪。 序列分析 TFX生产和抗性基因强烈表明， 肽是通过核糖体产生的（Breil等，1993，J.Bacteriol.，在 Press）。 tfxA结构基因的DNA序列和Edman 纯化肽的降解显示氨基酸序列 TFX的肽前体的第一个氨基酸是DIGGSRQGCVA。 这个肽序列 是由tfx基因后修饰的， 谷氨酰胺残基环化为UV吸收发色团。 的 这项工作的目的是确定这一结构， 发色团 通过反相HPLC分离TFX的两种异构体。 通过低分辨率FAB-MS确认分子的质量为 1037.4对于两种分子。 发色团的结构为 通过结合现代NMR技术，如COSY， TOCSY和HMQC是2-吡啶酮衍生物，其互变异构为 得到TFX异构体。 TFX的两种异构体也含有噻唑啉环 这是巯基之间闭环的结果， 半胱氨酸和甘氨酸的羰基。

Trifolitoxin (TFX) is a peptide bacteriocin produced by Rhizobium leguminosarum bv. trifoli T24 which is bacteriostatic toward most strains of R. leguminosarum and R. fredii. Sequence analysis of the TFX production and resistance genes strongly suggests that this peptide is produced ribosomally (Breil, et al. 1993, J. Bacteriol., in press). The DNA sequence of the tfxA structural gene and Edman degradation of the purified peptide show that the amino acid sequence of the peptide precursor of TFX is DIGGSRQGCVA. This peptide sequence is post-translationally modified by the tfx genes such that the glutamine residue is cyclized to a UV absorbing chromophore. The purpose of this work was to determine the structure of this chromophore. Two isomers of TFX were isolated by reverse phase HPLC. The mass of the molecules was confirmed by low resolution FAB-MS to be 1037.4 for both molecules. The structure of the chromophore was determined by a combination of modern NMR techniques, such as COSY, TOCSY and HMQC, to be a 2-pyridone derivative which tautomerizes to give TFX isomers. Both isomers of TFX also contain a thiazoline ring which is the result of ring closure between the sulfhydryl group of cysteine and the carbonyl of glycine.