Diversification of quiescin sulfhydryl oxidase in a preserved framework for redox relay.

BACKGROUND: The enzyme family Quiescin Sulfhydryl Oxidase (QSOX) is defined by the presence of an amino-terminal thioredoxin-fold (Trx) domain and a carboxy-terminal Erv family sulfhydryl oxidase domain. QSOX enzymes, which generate disulfide bonds and transfer them to substrate proteins, are present in a wide variety of eukaryotic species including metazoans and plants, but are absent from fungi. Plant and animal QSOXs differ in their active-site amino acid sequences and content of non-catalytic domains. The question arises, therefore, whether the Trx-Erv fusion has the same mechanistic significance in all QSOX enzymes, and whether shared features distinguish the functional domains of QSOX from other instances in which these domains occur independently. Through a study of QSOX phylogeny and an analysis of QSOX sequence diversity in light of recently determined three-dimensional structures, we sought insight into the origin and evolution of this multi-domain redox alliance. RESULTS: An updated collection of QSOX enzymes was used to confirm and refine the differences in domain composition and active-site sequence motif patterns of QSOXs belonging to various eukaryotic phyla. Beyond the expected phylogenetic distinction of animal and plant QSOX enzymes, trees based on individual redox-active QSOX domains show a particular distinction of the Trx domain early in plant evolution. A comparison of QSOX domains with Trx and Erv domains from outside the QSOX family revealed several sequence and structural features that clearly differentiate QSOXs from other enzymes containing either of these domains. Notably, these features, present in QSOXs of various phyla, localize to the interface between the Trx and Erv domains observed in structures of QSOX that model interdomain redox communication. CONCLUSIONS: The infrastructure for interdomain electron relay, previously identified for animal and parasite QSOXs, is found broadly across the QSOX family, including the plant enzymes. We conclude that the conserved three-dimensional framework of the QSOX catalytic domains accommodates lineage-specific differences and paralog diversification in the amino acid residues surrounding the redox-active cysteines. Our findings indicate that QSOX enzymes are characterized not just by the presence of the two defining domain folds but also by features that promote coordinated activity.

Specialization versus adaptation: two strategies employed by cyanophages to enhance their translation efficiencies.

Effective translation of the viral genome during the infection cycle most likely enhances its fitness. In this study, we reveal two different strategies employed by cyanophages, viruses infecting cyanobacteria, to enhance their translation efficiency. Cyanophages of the T7-like Podoviridae family adjust their GC content and codon usage to those of their hosts. In contrast, cyanophages of the T4-like Myoviridae family maintain genomes with low GC content, thus sometimes differing from that of their hosts. By introducing their own specific set of tRNAs, they appear to modulate the tRNA pools of hosts with tRNAs that fit the viral low GC preferred codons. We assessed the possible effects of those viral tRNAs on cyanophages and cyanobacterial genomes using the tRNA adaptation index, which measures the extent to which a given pool of tRNAs translates efficiently particular genes. We found a strong selective pressure to gain and maintain tRNAs that will boost translation of myoviral genes when infecting a high GC host, contrasted by a negligible effect on the host genes. Thus, myoviral tRNAs may represent an adaptive strategy to enhance fitness when infecting high GC hosts, thereby potentially broadening the spectrum of hosts while alleviating the need to adjust global parameters such as GC content for each specific host.

Phylogenetics and enzymology of plant quiescin sulfhydryl oxidase.

Quiescin Sulfhydryl Oxidase (QSOX), a catalyst of disulfide bond formation, is found in both plants and animals. Mammalian, avian, and trypanosomal QSOX enzymes have been studied in detail, but plant QSOX has yet to be characterized. Differences between plant and animal QSOXs in domain composition and active-site sequences raise the question of whether these QSOXs function by the same mechanism. We demonstrate that Arabidopsis thaliana QSOX produced in bacteria is folded and functional as a sulfhydryl oxidase but does not exhibit the interdomain electron transfer observed for its animal counterpart. Based on this finding, further exploration into the respective roles of the redox-active sites in plant QSOX and the reason for their concatenation is warranted.