Date

6-2015

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Biochemistry & Molecular Biology

Institution

Oregon Health & Science University

Abstract

The metabolic network for sulfide assimilation and trafficking in methanogens is largely unknown. To discover novel proteins required for these processes, bioinformatic methods were used to identify genes co-occurring with the protein biosynthesis enzyme SepCysS, which converts phosphoseryl-tRNACys to cysteinyl-tRNACys in nearly all methanogens. The analyses revealed three conserved protein-coding genes, each containing molecular signatures predicting functions in sulfur metabolism. All three genes were also identified in more than 50 strictly anaerobic bacterial genera from nine distinct phyla. Genotype-dependent growth and metabolite labeling experiments conducted in Methanosarcina acetivorans demonstrated that two of the proteins (MA1821 and MA1822) are essential to a novel homocysteine biosynthesis reaction, consuming aspartate-4-semialdehyde as a precursor. Mutational analysis confirmed the importance of several structural elements, including a conserved cysteine residue present in MA1821 and a predicted 4Fe-4S cluster-binding domain present in MA1822. Additional genotype-dependent growth experiments determined that the third protein (MA1715) is essential for growth with sulfide when present as the lone sulfur source at concentrations below 800 μM, indicating that MA1715 could be involved in mobilizing sulfur for the biosynthesis of cysteine and homocysteine. Moreover, phylogenetic analyses indicate that all three novel protein families were inherited vertically from the ancestral euryarchaeote along with SepCysS, suggesting that these four proteins comprise an ancient metabolic strategy for the assimilation of sulfide.

Identifier

doi:10.6083/M49Z93NF

School

School of Medicine

Available for download on Friday, June 22, 2018

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