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G673 could be a novel mutational hot spot for intragenic suppressors of pheS5 lesion in Escherichia coli.

ABSTRACT: The pheS5 Ts mutant of Escherichia coli defined by a G293 ? A293 transition, which is responsible for thermosensitive Phenylalanyl-tRNA synthetase has been well studied at both biochemical and molecular level but genetic analyses pertaining to suppressors of pheS5 were hard to come by. Here we have systematically analyzed a spectrum of Temperature-insensitive derivatives isolated from pheS5 Ts mutant and identified two intragenic suppressors affecting the same base pair coordinate G673 (pheS19 defines G673 ? T673 ; Gly225 ? Cys225 and pheS28 defines G673 ? C673 ; Gly225 ? Arg225). In fact in the third derivative, the intragenic suppressor originally named pheS43 (G673 ? C673 transversion) is virtually same as pheS28. In the fourth case, the very pheS5 lesion itself has got changed from A293 ? T293 (named pheS40). Cloning of pheS(+), pheS5, pheS5-pheS19, pheS5-pheS28 alleles into pBR322 and introduction of these clones into pheS5 mutant revealed that excess of double mutant protein is not at all good for the survival of cells at 42°C. These results clearly indicate a pivotal role for Gly225 in the structural/functional integrity of alpha subunit of E. coli PheRS enzyme and it is proposed that G673 might define a hot spot for intragenic suppressors of pheS5.

SUBMITTER: Ponmani T

PROVIDER: S-EPMC4082710 | biostudies-literature | 2014 Jun

REPOSITORIES: biostudies-literature

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G673 could be a novel mutational hot spot for intragenic suppressors of pheS5 lesion in Escherichia coli.

Ponmani Thangaraj T Munavar M Hussain MH

MicrobiologyOpen 20140508 3

The pheS5 Ts mutant of Escherichia coli defined by a G293 → A293 transition, which is responsible for thermosensitive Phenylalanyl-tRNA synthetase has been well studied at both biochemical and molecular level but genetic analyses pertaining to suppressors of pheS5 were hard to come by. Here we have systematically analyzed a spectrum of Temperature-insensitive derivatives isolated from pheS5 Ts mutant and identified two intragenic suppressors affecting the same base pair coordinate G673 (pheS19 d ...[more]

PMID: 24811065

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Project description:dGTP starvation, a newly discovered phenomenon in which Escherichia coli cells are starved specifically for the DNA precursor dGTP, leads to impaired growth and, ultimately, cell death. Phenomenologically, it represents an example of nutritionally induced unbalanced growth: cell mass amplifies normally as dictated by the nutritional status of the medium, but DNA content growth is specifically impaired. The other known example of such a condition, thymineless death (TLD), involves starvation for the DNA precursor dTTP, which has been found to have important chemotherapeutic applications. Experimentally, dGTP starvation is induced by depriving an E. coligpt optA1 strain of its required purine source, hypoxanthine. In our studies of this phenomenon, we noted the emergence of a relatively high frequency of suppressor mutants that proved resistant to the treatment. To study such suppressors, we used next-generation sequencing on a collection of independently obtained mutants. A significant fraction was found to carry a defect in the PurR transcriptional repressor, controlling de novo purine biosynthesis, or in its downstream purEK operon. Thus, upregulation of de novo purine biosynthesis appears to be a major mode of overcoming the lethal effects of dGTP starvation. In addition, another large fraction of the suppressors contained a large tandem duplication of a 250- to 300-kb genomic region that included the purEK operon as well as the acrAB-encoded multidrug efflux system. Thus, the suppressive effects of the duplications could potentially involve beneficial effects of a number of genes/operons within the amplified regions.IMPORTANCE Concentrations of the four precursors for DNA synthesis (2'-deoxynucleoside-5'-triphosphates [dNTPs]) are critical for both the speed of DNA replication and its accuracy. Previously, we investigated consequences of dGTP starvation, where the DNA precursor dGTP was specifically reduced to a low level. Under this condition, E. coli cells continued cell growth but eventually developed a DNA replication defect, leading to cell death due to formation of unresolvable DNA structures. Nevertheless, dGTP-starved cultures eventually resumed growth due to the appearance of resistant mutants. Here, we used whole-genome DNA sequencing to identify the responsible suppressor mutations. We show that the majority of suppressors can circumvent death by upregulating purine de novo biosynthesis, leading to restoration of dGTP to acceptable levels.

Project description:To report a 3-generation white family clinically diagnosed variably with Wagner, Stickler, and Jansen syndromes and screened for sequence variants in the COL2A1 and CSPG2 genes. Wagner syndrome is an autosomal dominant vitreoretinopathy with a predisposition to retinal detachment and cataracts. It has significant phenotypic overlap with allelic Jansen syndrome and ocular Stickler syndrome type 1. Sticker syndrome type 1 maps to chromosome 12q13.11-q13.2, with associated COL2A1 gene mutations. Wagner syndrome maps to chromosome 5q13-q14 and is associated with mutations in CSPG2 encoding versican, a proteoglycan present in human vitreous.Genomic DNA samples derived from venous blood were collected from all family members. Complete sequencing of COL2A1 was performed on a proband. Primers for polymerase chain reaction and sequencing were designed to cover all exon and intron-exon boundaries. Direct sequencing of CSPG2 was performed on all family member samples.No detectable COL2A1 mutations were noted, making the diagnosis of ocular Stickler syndrome highly unlikely for this family. A unique base pair substitution (c.9265 + 1G>T) in intron 8 of the CSPG2 gene cosegregating with disease status was identified. This mutation occurred in a highly conserved previously reported splice site with a similar base pair substitution (G>A). Direct sequencing of this splice site mutation in 107 unrelated external controls revealed no variants, supporting the rarity of this base pair change and its causation in Wagner syndrome. This novel base pair substitution is thought to cause the deletion of exon 8 and formation of a truncated protein product.Mutation screening of CSPG2 in autosomal dominant vitreoretinopathy families is important for accurate diagnosis.This study underscores the importance of obtaining extensive pedigree information and comparative ophthalmologic clinical information, as the phenotypic findings may vary greatly among independent family members. The study also affirms the paradigm shift from diagnosis assignment based on eponyms to that based on gene mutation type.

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G673 could be a novel mutational hot spot for intragenic suppressors of pheS5 lesion in Escherichia coli.

Publications

G673 could be a novel mutational hot spot for intragenic suppressors of pheS5 lesion in Escherichia coli.

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