Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci.
Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci.
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DOI：
10.1099/00221287-144-3-655
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发表时间：
1998-03
期刊：
影响因子：
1.5
通讯作者：
T. Tao;P. Grulich;L. M. Kucharski;R. Smith;M. Maguire
中科院分区：
文献类型：
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作者：
T. Tao;P. Grulich;L. M. Kucharski;R. Smith;M. Maguire
鼠伤寒沙门氏菌具有三种不同的Mg 2+转运系统，组成型高容量CorA转运蛋白和两种P型ATP酶，MgtA和MgtB，其转录被生长培养基中正常浓度的Mg 2+抑制。后一个Mg（2+）转运ATP酶是双基因操纵子mgtCB的一部分，其中mgtC编码一个未知功能的23 kDa蛋白。使用mgtA和mgtCB的启动子区域与luxAB的融合的转录调节显示出双相时间和Mg 2+浓度依赖性。转移到含确定浓度的Mg 2+的氮基本培养基后1和6小时之间，转录增加约200倍的mgtCB和高达400倍的mgtA，每一个与Mg 2+的半最大依赖性为0.5 mM。继续孵育揭示了第二阶段的转录增加，高达2000倍的mgtCB和高达10,000倍的mgtA。对于mgtCB，转移至确定培养基后6 - 9 h出现二次增加，但对于mgtA，二次增加在12 - 24 h之间，并且对0.01 mM的Mg 2+具有明显的半最大依赖性。在与任一系统孵育8 - 24 h之间，观察到阳离子摄取量同时增加至少1000倍，表明转录增加之后是大量新合成的转运蛋白功能性掺入膜中。Mg 2+的转录调控不依赖于由rpoS编码的功能性静止相σ因子，但它依赖于功能性phoPQ双组分调控系统的存在。尽管mgtCB完全依赖于通过phoPQ的调节，但在phoP或phoQ中携带突变的菌株中，mgtA转录的次级晚期Mg（2+）依赖性阶段仍然明显，尽管大幅减少。几种二价阳离子阻断了由Mg 2+浓度降低引起的转录增加的早期阶段，包括抑制Mg 2+摄取的阳离子（Co2+、Ni 2+和Mn 2+）和不抑制Mg 2+摄取的阳离子（Ca 2+和Zn 2+）。与此相反，转录增加的第二个后期没有很好地阻止任何阳离子，除了那些抑制摄取。总体而言，数据表明，至少有两个不同的机制，转录调控的mgtA和mgtCB基因座存在。
Salmonella typhimurium has three distinct Mg2+ transport systems, the constitutive high-capacity CorA transporter and two P-type ATPases, MgtA and MgtB, whose transcription is repressed by normal concentrations of Mg2+ in the growth medium. The latter Mg(2+)-transporting ATPase is part of a two-gene operon, mgtCB, with mgtC encoding a 23 kDa protein of unknown function. Transcriptional regulation using fusions of the promoter regions of mgtA and mgtCB to luxAB showed a biphasic time and Mg2+ concentration dependence. Between 1 and 6 h after transfer to nitrogen minimal medium containing defined concentrations of Mg2+, transcription increased about 200-fold for mgtCB and up to 400-fold for mgtA, each with a half-maximal dependence on Mg2+ of 0.5 mM. Continued incubation revealed a second phase of increased transcription, up to 2000-fold for mgtCB and up to 10,000-fold for mgtA. This secondary increase occurred between 6 and 9 h after transfer to defined medium for mgtCB but between 12 and 24 h for mgtA and had a distinct half-maximal dependence for Mg2+ of 0.01 mM. A concomitant increase of at least 1000-fold in uptake of cation was seen between 8 and 24 h incubation with either system, showing that the transcriptional increase was followed by functional incorporation of large amounts of the newly synthesized transporter into the membrane. Regulation of transcription by Mg2+ was not dependent on a functional stationary-phase sigma factor encoded by rpoS, but it was dependent on the presence of a functional phoPQ two-component regulatory system. Whereas mgtCB was completely dependent on regulation via phoPQ, the secondary late Mg(2+)-dependent phase of mgtA transcription was still evident in strains carrying a mutation in either phoP or phoQ, albeit substantially diminished. Several divalent cations blocked the early phase of the increase in transcription elicited by the decrease in Mg2+ concentration, including cations that inhibit Mg2+ uptake (Co2+, Ni2+ and Mn2+) and those which do not (Ca2+ and Zn2+). In contrast, the second later phase of the transcriptional increase was not well blocked by any cation except those which inhibit uptake. Overall, the data suggest that at least two distinct mechanisms for transcriptional regulation of the mgtA and mgtCB loci exist.