Knight T J, Durbin R D, Langston-Unkefer P J
J Bacteriol. 1986 Apr;166(1):224-9. doi: 10.1128/jb.166.1.224-229.1986.
Selected pathovars of Pseudomonas syringae produce an extracellular phytotoxin, tabtoxinine-beta-lactam, that irreversibly inhibits its known physiological target, glutamine synthetase (GS). Pseudomonas syringae subsp. "tabaci" retains significant amounts of glutamine synthetase activity during toxin production in culture. As part of our investigation of the self-protection mechanism(s) used by these pathovars, we have determined that GS becomes adenylylated after toxin production is initiated and that the serine released from the zinc-activated hydrolysis of tabtoxin is a factor in the initiation of this adenylylation. The adenylylation state of this GS was estimated to range from E5.0-7.5. The irreversible inactivation by tabtoxinine-beta-lactam of unadenylylated and adenylylated glutamine synthetase purified from P. syringae subsp. "tabaci" was investigated. Adenylylated GS was inactivated by tabtoxinine-beta-lactam at a slower rate than was unadenylylated enzyme. Adenylylated GS (E7.5-10.5) was significantly protected from this inactivation in the presence of the enzyme effectors, AMP, Ala, Gly, His, and Ser. Thus, the combination of the adenylylation of GS after toxin production is initiated and the presence of the enzyme effectors in vivo could provide part of the self-protection mechanism used by subsp. "tabaci".
丁香假单胞菌的某些致病变种会产生一种细胞外植物毒素——烟草毒素β-内酰胺，它会不可逆地抑制其已知的生理靶点谷氨酰胺合成酶（GS）。丁香假单胞菌烟草致病变种在培养过程中产生毒素时会保留大量的谷氨酰胺合成酶活性。作为我们对这些致病变种所采用的自我保护机制研究的一部分，我们已确定在毒素产生开始后GS会被腺苷酸化，并且从烟草毒素的锌激活水解中释放的丝氨酸是这种腺苷酸化起始的一个因素。这种GS的腺苷酸化状态估计在E5.0 - 7.5范围内。研究了从丁香假单胞菌烟草致病变种中纯化的未腺苷酸化和腺苷酸化的谷氨酰胺合成酶被烟草毒素β-内酰胺不可逆失活的情况。腺苷酸化的GS被烟草毒素β-内酰胺失活的速率比未腺苷酸化的酶慢。在酶效应物AMP、丙氨酸、甘氨酸、组氨酸和丝氨酸存在的情况下，腺苷酸化的GS（E7.5 - 10.5）能显著免受这种失活的影响。因此，毒素产生开始后GS的腺苷酸化以及体内酶效应物的存在可能构成烟草致病变种所采用的自我保护机制的一部分。
