The allocation of carbon and nitrogen resources to the synthesis of plant proteins, carbohydrates, and lipids is complex and under the control of many genes; much remains to be understood about this process. QQS (Qua-Quine Starch; At3g30720), an orphan gene unique to Arabidopsis thaliana, regulates metabolic processes affecting carbon and nitrogen partitioning among proteins and carbohydrates, modulating leaf and seed composition in Arabidopsis and soybean. Here the universality of QQS function in modulating carbon and nitrogen allocation is exemplified by a series of transgenic experiments. We show that ectopic expression of QQS increases soybean protein independent of the genetic background and original protein content of the cultivar. Furthermore, transgenic QQS expression increases the protein content of maize, a C4 species (a species that uses 4-carbon photosynthesis), and rice, a proteinpoor agronomic crop, both highly divergent from Arabidopsis. We determine that QQS protein binds to the transcriptional regulator AtNF-YC4 (Arabidopsis nuclear factor Y, subunit C4). Overexpression of AtNF-YC4 in Arabidopsis mimics the QQS-overexpression phenotype, increasing protein and decreasing starch levels. NF-YC, a component of the NF-Y complex, is conserved across eukaryotes. The NF-YC4 homologs of soybean, rice, and maize also bind to QQS, which provides an explanation of how QQS can act in species where it does not occur endogenously. These findings are, to our knowledge, the first insight into the mechanism of action of QQS in modulating carbon and nitrogen allocation across species. They have major implications for the emergence and function of orphan genes, and identify a nontransgenic strategy for modulating protein levels in crop species, a trait of great agronomic significance.QQS | NF-YC4 | carbon allocation | nitrogen allocation | orphan C arbon and nitrogen allocation to plant proteins, carbohydrates, and lipids is not controlled by a single gene but by many (1). Most of the enzymes promoting accumulation of these products have been identified; however, much less is understood about the mechanisms that regulate this complex metabolic network (2-8).Arabidopsis thaliana QQS (Qua-Quine Starch; At3g30720) lacks sequence similarity to any other protein-coding genes, and is considered an orphan gene that has arisen de novo from noncoding sequence since the divergence of A. thaliana from other species (9, 10). Although orphans typically comprise 2-8% of the genome of eukaryotic and prokaryotic species, their origin and biological function have not been well-explored (11)(12)(13)(14). Proteins encoded by some orphan genes provide a defensive capability by binding to a receptor of a predator organism (11). In contrast, QQS action is endogenous (3): Overexpression of QQS in Arabidopsis increases total protein content and decreases total starch content in leaf, whereas down-regulation of QQS has the converse effect. The increased starch content in QQS RNAi (RNA interference) mutants is due to increased starch accumu...