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Source: Journal of Biological Chemistry Authors: Kai Xu, Tao Xia 報告者 : 吴广霞 PowerPoint Presentation
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A novel plant vacuolar Na+/H+ antiporter gene evolved by DNA shuffling confers improved salt tolerance in yeast. Source: Journal of Biological Chemistry Authors: Kai Xu, Tao Xia 報告者 : 吴广霞. ABSTRACT.

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slide1

A novel plant vacuolar Na+/H+ antiporter gene evolved by DNA shuffling confers improved salt tolerance in yeast

Source: Journal of Biological Chemistry

Authors: Kai Xu,

Tao Xia

報告者: 吴广霞

abstract
ABSTRACT
  • Plant vacuolar Na+/H+ antiporters play important roles in maintaining cellular ion homeostasis and mediating the transport of Na+ out of the cytosol and into the vacuole. Vacuolar antiporters have been shown to play significant roles in salt tolerance, however the relatively low Vmax of the Na+/H+ exchange of the Na+/H+ antiporters identified could limit its application in the molecular breeding of salt tolerant crops. In this study, we applied DNA shuffling methodology to generate and recombine the mutations of Arabidopsis thaliana vacuolar Na+/H+ antiporter gene AtNHX1. Screening using a large scale yeast complementation system identified AtNHXS1, a novel Na+/H+ antiporter. Expression of AtNHXS1 in yeast showed that the antiporter localized to the vacuolar membrane and that its expression improved the tolerance of yeast to NaCl, KCl, LiCl and hygromycin B. Measurements of the ion transport activity across the intact yeast vacuole demonstrated that the AtNHXS1 protein showed higher Na+/H+ exchange activity and a slightly improved K+/H+ exchange activity.
slide3
DNA改组
  • DNA改组是DNA分子的体外重组,是基因在分子水平上有性重组( sexual recombination)。通过改变单个基因(或基因家族,gene family) 原有核苷酸序列,创造新基因,并赋予表达产物以新功能。实际上,该技术是一种分子水平上的定向进化(directed evolution)。因此也称为 子 育 种 ( molecular-breeding)。在创造新基因的过程中,要设法产生各种变异。
  • DNA改组的原理

DNA改组实际上是依赖于PCR的体外诱变技术。它是将单个基因或相关基因家族的靶序列通过物理或化学方法随机片段化,由于这些小片段之间具有一定的同源性,通过无引物PCR和有引物PCR组装成全长的嵌合体基因即嵌合体文库。然后对嵌合体文库进行高通量或超高通量的筛选,选择具有改进功能或全新功能的突变体作为下轮 DNA改组的模板,重复上述步骤,进行多轮改组和高通量的筛选,直到获得理想的突变体。

experimental procedures
EXPERIMENTAL PROCEDURES

Yeast strains

1

TY001

W303-1B

Δena1-2 (Δena1-2::HIS3)

Δena1-2

  • Δena1-4Δnhx1 (Δena1-4::HIS3,
  • Δnhx1::TRP1)

Δena1-4Δnhx1

Δnhx1::TRP1

NHX1 deletion mutant

TY001 (Δnhx1::TRP1)

slide6

pGM-T-AtNHX1

PCR amplification

AtNHX1

2µg products

DNase I

digestion 2 min

100 and 200 bp fragments

purified fragments (1µg)

primerless PCR

Reassemble

full-length sequences

primer PCR

a single band with the correct size

DNA shuffling of AtNHX1

2

slide8

The construction of shuffled gene library

and screening for high salt tolerance yeast clones

3

The reassembled fragment of the correct size

ligation

pYPGE15

transformation

lithium acetate method

Δena1-4::HIS3 ,Δnhx1::TRP1

Early screening

30 ℃for 4 days

APG selective medium

100mM NaCl

OD600=0.5, 5µl cells

the second selection

30 ℃for 4 days

APG , 125mM NaCl

yeast transformation drop tests and intracellular ion content determination
Yeast Transformation, drop tests and intracellular ion content determination

4

pYPGE15- AtNHX1/ AtNHXS1

pYPGE15

  • For drop tests, the cells of different strains were harvested by centrifugation and resuspended into APG medium and adjusted the OD600 to 0.5. Serial 5 times dilutions were made and 5µl of the cells were loaded onto APG medium with different salt supplements.

transformation

Δena1-4::HIS3,Δnhx1::TRP1

W303-1B,

Δena1-2::HIS3

Δena1-4::HIS3,Δnhx1::TRP1

atnhx1 gfp localization and con focal microscopy
AtNHX1-GFP localization and con-focal microscopy

5

pYPGE15-GFP3 TY001

pYPGE15- AtNHX1-GFP3 TY001

pYPGE15- AtNHXS1-GFP3 TY001

discussion
DISCUSSION
  • An appropriate digested fragment length is crucial for a successful shuffling.
  • A suitable selective model and expression system are key factors for successful mutant screening.
discussion1
DISCUSSION
  • Y241P and F242L would not play any role in cation or proton transport.
  • As proline is an helix breaker which can disrupt the alpha-helices’ structure, it is possible that the L29P or S158P substitutions resulted in the alteration of Na+/H+ antiporter structure, favoring Na+/H+ exchange.
conclusions
CONCLUSIONS
  • 1.This shuffled clone displayed enhanced tolerance to NaCl, KCl, LiCl and hygromycin B .
  • 2. The amino acids mutations and C-terminus partial deletion in AtNHXS1 did not alter the localization of the antiporter, and increased the antiporter Na+/H+ exchange activity, thus enabling a higher NaCl tolerance.
conclusions1
CONCLUSIONS
  • 3.AtNHXS1 displayed a lesser increment in the Vmax of K+/H+ exchange and there was some increase in yeast KCl tolerance.
  • 4.The C-terminus deletion and the mutations in the putative cation transport path and the mutations in the cation binding domain of the Na+/H+ antiporter increased the Na+/H+ exchange activity and the Na+ selectivity, whereas decreased the Na+ and K+ affinity.