Expression of the PitA phosphate/metal transporter ofEscherichia coliis responsive to zinc and inorganic phosphate levels

Abstract

Escherichia coli possesses two major systems for inorganic phosphate (P(i)) uptake. The Pst system (pstSCAB) is inducible by low phosphate concentrations whereas the low-affinity transporter (pitA) has been described as constitutively expressed. PitA catalyses transport of metal [Mg(II), Ca(II)]-phosphate complexes, and mutations in pitA confer Zn(II) resistance. Here we report that pitA transcription is not constitutive; activity of a single-copy pitA-lacZ transcriptional fusion (monolysogen) was maximal at high extracellular Zn(II) (150 microM), in the absence of added P(i), and in a well-defined pitA mutant strain. Intracellular zinc levels were unaffected by adding Zn(II) to the medium for both the wild-type and mutant strains. However, in the wild-type strain, Mg levels (per gram of dry biomass) fell by eightfold in cells grown with added Zn(II) and by 20-fold when Zn(II) and P(i) were added to cultures. Mutation of pitA reduced the effects of external Zn(II) and phosphate levels on Mg pools, consistent with competition or inhibition by Zn(II) of PitA. The mechanism of pitA regulation by extracellular Zn(II) and P(i) is unknown but appears not to involve Fur or other well-characterized regulators.

Induction of the Pho Regulon Suppresses the Growth Defect of an Escherichia coli sgrS Mutant, Connecting Phosphate Metabolism to the Glucose-Phosphate Stress Response

“…Normally, UhpT takes up glucose-6-phosphate in exchange for excretion of P i . However, UhpT also has a low affinity for P i and can work in the opposite direction, although it cannot support growth of a pst pitA mutant when grown on P i (18,25,43,48,68). We tested the hypothesis that the pitA mutation suppressed the ␣MG growth defect of the sgrS mutant by promoting UhpT-dependent efflux of ␣MG-6-phosphate.…”

“…In a pitA mutant, P i primarily enters the cell through the high-affinity transporter and Pho regulon member PstSCAB (18,20,25,48,66). However, P i may also be able to enter pitA cells through the transporter UhpT (25,43,68). Normally, UhpT takes up glucose-6-phosphate in exchange for excretion of P i .…”

“…PitA is a low-affinity transporter; when P i is plentiful, it is cotransported into the cell by PitA, along with divalent cations (6,25,47,48,57,67). One of the cations transported by PitA is Zn 2ϩ ; while pitA is always expressed at some level, its transcription is upregulated in response to the combination of low P i and high Zn 2ϩ concentrations (6,25). pstSCAB encodes the other major P i transporter, which has a high affinity for P i .…”

Functional Characterization of Synechocystis sp. Strain PCC 6803 pst1 and pst2 Gene Clusters Reveals a Novel Strategy for Phosphate Uptake in a Freshwater Cyanobacterium

“…In E. coli, uptake utilizes both a low-affinity permease, the Pit system (54) [with uptake of P i being reliant on cotransport with divalent metal cations such as Mg(II) or Ca(II) through the formation of a soluble, neutral metal-phosphate complex, which is then the transported species (28,49)] and a high-affinity Pst transport system (52). The Pst transporter comprises a periplasmic P i -binding protein (PstS), two integral membrane proteins (PstA and PstC), and an ATPbinding protein (PstB) (10,44).…”

Bacterial Phosphate Homeostasis: Role of Phosphate Transporters

“…Pit 시스템은 많은 세균에 서 pitA 유전자 하나에 의해 생산되는 막횡단 단백질이며, 유전 자 전사는 pst 오페론과 달리 Pi 농도 변화에 의해 변화하지 않는 것으로 알려졌다 . 최근 E. coli를 상대로 한 연구 에서는 pitA-lacZ 리포터의 활성이 높은 농도의 Zn 2+ 와 낮은 Pi 농도에서 증가하는 것을 확인했으나, 아직 정확한 기작은 알려 지지 않고 있다 (Jackson et al, 2008).…”