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Spectral Modulation Using Direct Mutagenesis Case Study

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1.2.1 Spectral modulation using site-directed mutagenesis
Spectral tuning through the use of random PCR mutagenesis has been previously attempted (Kim et al. 2008). From the mutants obtained, 8 showed red-shifted absorption maxima and 12 mutants are blue-shifted compared to the wild-type (Kim et al. 2008). However, most of these mutants showed a loss of pumping activity.Various other red-shifted mutants of PR and GR have been generated (Martin K.M. Engqvist, 2015) (Srividya Ganapathy, 2015). The most conventional method of generating retinal binding pocket mutants employs mismatch PCR. In the study by Ganapathy et. al, a red-shifted double mutant PR-D212N/F234S(PR-DNFS) was produced and tested with several retinal analogs for the …show more content…

al. 2015) since retinal binding and functional group interaction is delicate and may vary with protein mutations.
In the current study, we aimed to develop a directed evolution assay, which would allow us to generate and select mutants with red-shifted absorbance and conserved proton pumping ability simultaneously, using E. coli as a host organism. The main step in designing a directed evolution system in E. coli warrants a short overview of the relevant characteristics of the bacteria, which can be affected by a PR-generated light-driven PMF, and used for selection.
Previous studies have shown that PRs can provide a selective advantage to E. coli only under conditions of starvation or respiratory stress. When inhibited by depleting oxygen or by the respiratory poison azide, E. coli cells expressing proteorhodopsin are shown to become light-powered in a sense that illumination of the cells with green light creates a proton motive force capable of driving the flagellar motor (Walter et al. 2007) (Murray J. Tipping, 2013). This yields motile bacteria under respiratory stress where they would otherwise remain stagnant (Walter et al. 2007). Azide inhibits cytochrome oxidase and, thus, proton extrusion by the respiratory chain, stopping the flagellar motor (Gabel and Berg 2003).
In this research, we used sodium azide as a respiratory inhibitor of E. coli.

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