Metzger S, Schreiber G, Aizenman E, Cashel M, Glaser G
Department of Cellular Biochemistry, Hebrew University Hadassah Medical School, Jerusalem, Israel.
J Biol Chem. 1989 Dec 15;264(35):21146-52.
The most widely studied "relaxed" mutant of the relA locus, the relA1 allele, is shown here to consist of an IS2 insertion between the 85th and 86th codons of the otherwise wild-type relA structural gene, which normally encodes a 743-amino acid (84 kDa) protein. The RelA protein is a ribosome-dependent ATP:GTP (GDP) pyrophosphoryltransferase that is activated during the stringent response to amino acid starvation and thereby occasions the accumulation of guanosine 3',5'-bispyrophosphate (ppGpp). We propose that the IS2 insertion functionally splits the RelA protein into two (alpha and beta) peptide fragments which can complement each other in trans to yield residual ppGpp synthetic activity; neither fragment shows this activity when expressed alone. Cell strains with a single copy relA null allele show physiological behavior that is much the same as relA1 mutant strains. Both relA1 and relA null strains accumulate ppGpp during glucose starvation and do not accumulate ppGpp during the stringent response. The presence of ppGpp in verifiable relA null strains is interpreted as unequivocal evidence for an alternate route of ppGpp synthesis that exists in addition to the relA-dependent reaction.
relA基因座中研究最广泛的“松弛”突变体relA1等位基因，在此显示为由野生型relA结构基因第85和86密码子之间的IS2插入组成，该结构基因通常编码一种743个氨基酸（84 kDa）的蛋白质。RelA蛋白是一种核糖体依赖性ATP：GTP（GDP）焦磷酸转移酶，在对氨基酸饥饿的严紧反应中被激活，从而导致鸟苷3'，5'-双焦磷酸（ppGpp）的积累。我们提出，IS2插入在功能上将RelA蛋白分裂成两个（α和β）肽片段，它们可以反式互补以产生残余的ppGpp合成活性；单独表达时，两个片段均不显示这种活性。具有单拷贝relA无效等位基因的细胞株表现出与relA1突变株非常相似的生理行为。relA1和relA无效株在葡萄糖饥饿期间积累ppGpp，而在严紧反应期间不积累ppGpp。在可验证的relA无效株中ppGpp的存在被解释为除了relA依赖性反应之外还存在ppGpp合成替代途径的确凿证据。
