先前研究已知在Saccharomyces cerevisiae中一個VAS1基因(ScVAS1)可以藉由不同的轉錄以及轉譯起始點做出兩種valyl-tRNA synthetase (ValRS)的異構型蛋白質，這兩種異構型蛋白質分別具有粒線體以及細胞質功能。我們發現在C. albicans及Y. lipolytica中唯一的VAS1基因(CaVAS1及YlVAS1)也是由類似的機制做出兩種異構型的蛋白質，這顯示在酵母菌中這種由一個基因做出兩種異構形的ValRS是普遍存在的。此外也發現在酵母菌S. pombe的細胞核染色體中具有SpVAS1以及SpVAS2兩個不同的基因，其中SpValRS2多了一段細菌ValRS沒有的附加區段，且能夠互補ScVAS1剔除菌株之細胞質功能。雖然SpValRS1缺乏附加區段，但是其在胺基端多了一段粒線體標的訊號(mitochondrial targeting signal)，功能試驗的結果顯示SpValRS1不能互補ScVAS1剔除菌株之粒線體功能。除此之外， SpVAS1及SpVAS2皆為S. pombe生長所必須之基因，因此在S. pombe中 SpValRS1很可能是粒線體的蛋白質，而SpValRS2是細胞質的蛋白質。基於這些發現我們推測SpVAS1及SpVAS2很可能是酵母菌ValRS演化的早期狀態，在演化的過程中一些酵母菌丟棄了其中一個ValRS基因，而另一個則衍生出雙重功能。 Previous studies showed that a single yeast gene, ScVAS1, specifies both the cytosolic and mitochondrial forms of valyl-tRNA synthetase (ValRS) through alternative transcription and translation. We show here that CaVAS1 and YlVAS1, the only ValRS genes in Candida albicans and Yarrowia lipolytica, respectively, follow a similar mechanism for expression of their enzyme isoforms. It appears that one gene encoding two enzyme isoforms is a universal mechanism in yeast. Interestingly, there are two homologous genes, SpVAS1 and SpVAS2, in the chromosomes of fission yeast Schizosaccharomyces pombe. Our data showed that SpValRS2 is a cytosolic protein that possesses an N-terminal appended domain and can complement a ScVAS1 knockout strain. Unexpectedly, while SpValRS1 carries a typical mitochondria targeting signal (MTS) and lacks an appended domain, it can not complement the mitochondrial function of a ScVAS1 knockout strain. In addition, we show that both SpVAS1 and SpVAS2 are essential genes in S. pombe. It is likely that SpValRS1 is a mitochondrial protein, while SpValRS2 is a cytosolic protein in S. pombe. It is therefore likely that SpVAS1 and SpVAS2 represent an early state of the evolutionary process of ValRS. During evolution, one of the genes is lost, while the other has evolved a bifunctional preperty.
