Document Type


Degree Name



Dept. of Biochemistry & Molecular Biology


Oregon Health & Science University


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.




School of Medicine



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.