Metabolic regulation and genetic tools for bacterial neutral lipid production.

Abstract

Wax esters are neutral lipids that find diverse applications as cosmetics components, lubricants and fuels. Most of the present production of wax esters is derived from non-renewable fossil oil. Given the diversity of uses, essentiality and their burgeoning demand, it is prudent to develop renewable alternatives to current methods of production. The recent discovery of wax biosynthesis in a niche of bacteria has created much interest in exploring these organisms both for their biochemical and biotechnological significance. In the first section of this thesis, I describe efforts to understand the metabolic regulation involved in bacterial wax biosynthesis. Wax biosynthesis is induced in bacteria like Marinobacter aquaeolei VT8 and Psychrobacter cryohalolentis K5 when the organisms are starved of nitrogen nutrients. It is of interest to know how bacterial metabolism redirects its carbon flow to wax accumulation in the absence of fixed nitrogen. We reasoned that there would be elevated expression of proteins that formed the metabolic route leading to wax formation. Therefore, it would be possible to develop a hypothesis about the metabolic pathway that leads to wax esters if differences in protein expression could be found out between cells before and during wax biosynthesis. Proteomic experiments were performed to compare the cellular protein composition before and after wax accumulation, resulting in a catalogue of proteins differentially expressed between the two metabolic states. These proteins represented the metabolic changes during wax biosynthesis. To fill in key gaps in the proteomics derived information and aid in further development of the hypothesis, quantitative methods of analyzing gene expression at the transcriptional level were adopted. The current state of progress from these studies is discussed. In the second section of this thesis, I explain the development of genetic tools that enable recombinant gene expression in select wax ester producing bacteria. A self-transmissible plasmid vector and a conjugal method of DNA transfer were adopted to deliver non-native DNA into the target bacterial strain Psychrobacter arcticus 273-4 ii Abstract Wax esters are neutral lipids that find diverse applications as cosmetics components, lubricants and fuels. Most of the present production of wax esters is derived from non-renewable fossil oil. Given the diversity of uses, essentiality and their burgeoning demand, it is prudent to develop renewable alternatives to current methods of production. The recent discovery of wax biosynthesis in a niche of bacteria has created much interest in exploring these organisms both for their biochemical and biotechnological significance. In the first section of this thesis, I describe efforts to understand the metabolic regulation involved in bacterial wax biosynthesis. Wax biosynthesis is induced in bacteria like Marinobacter aquaeolei VT8 and Psychrobacter cryohalolentis K5 when the organisms are starved of nitrogen nutrients. It is of interest to know how bacterial metabolism redirects its carbon flow to wax accumulation in the absence of fixed nitrogen. We reasoned that there would be elevated expression of proteins that formed the metabolic route leading to wax formation. Therefore, it would be possible to develop a hypothesis about the metabolic pathway that leads to wax esters if differences in protein expression could be found out between cells before and during wax biosynthesis. Proteomic experiments were performed to compare the cellular protein composition before and after wax accumulation, resulting in a catalogue of proteins differentially expressed between the two metabolic states. These proteins represented the metabolic changes during wax biosynthesis. To fill in key gaps in the proteomics derived information and aid in further development of the hypothesis, quantitative methods of analyzing gene expression at the transcriptional level were adopted. The current state of progress from these studies is discussed. In the second section of this thesis, I explain the development of genetic tools that enable recombinant gene expression in select wax ester producing bacteria. A self-transmissible plasmid vector and a conjugal method of DNA transfer were adopted to deliver non-native DNA into the target bacterial strain Psychrobacter arcticus 273-4 from a donor Escherichia coli strain. In plasmid-bearing colonies of P. arcticus 273-4, a puzzling phenomenon was observed where the circular plasmid delivered into the bacteria had increased in size, potentially capable of undermining recombinant gene expression efforts. However, the cause was identified to be a transposon insertion event originating from the donor E. coli strain which harbored the transposon, a parasitic genetic element. A 38 base pair sequence, previously reported to be effective against transposon insertions, was incorporated into the plasmid vector. Testing the efficacy of the solution indicated that presence of the introduced sequence was sufficient to prevent transposon insertion events. In the final section of this thesis, I describe the construction of plasmid vectors that would enable the biosynthesis of myrcene, a monoterpene hydrocarbon, in recombinant E. coli. This system is intended to be a prototype for monoterpene biosynthesis in the cyanobacterium, Anabaena variabilis. One of the design goals was to assemble two native and one mutated E. coli genes in a single plasmid vector to enable biosynthesis of geranyl pyrophosphate (GPP), the precursor for myrcene. The three genes were assembled into an operon, a gene organization principle observed in bacteria where the expression of contiguous genes is driven by a single promoter. During an intermediate step, an issue perceived to be poor protein expression from one of the genes was resolved. The plasmid vector pPCRISPDSX11 Col 1 contained the final construct of the synthetic operon. Electrophoretic analysis provided visual evidence that all three genes in the vector were being expressed. This would imply that the precursor GPP is being produced in E. coli, thereby making significant progress towards the stated aim.