Singleton sequence ORFans are orphan ORFs (open reading frames) that have no detectable sequence similarity to any other sequence in the databases. ORFans are of particular interest not only as evolutionary puzzles but also because we can learn little about them using bioinformatics tools. Here, we present a first systematic analysis of singleton ORFans in the first 60 fully sequenced microbial genomes. We show that although ORFans have been underemphasized, the number of ORFans is steadily growing, currently accounting for 23,634 sequences. At the same time, the percentage of ORFans as a fraction of all sequences is slowly diminishing, and is currently about 14%. Short ORFans comprise about 61% of all ORFans. The abundance of short ORFans may be due to a yet unexplained artifact. The data also suggest that the number of longer ORFans may soon diminish as more genomes of closely related organisms become available. To better address the questions about the functions and origins of ORFans, we propose to focus further studies on the longer ORFans, with emphasis on three new types of ORFans: ORFan modules, paralogous ORFans, and orthologous ORFans. We conclude that the large number of ORFans reflects an intrinsic property of the genetic material not yet fully understood. Further computational and experimental studies aimed at understanding Nature's protein diversity should also include ORFans.
Copyright 2003 Wiley-Liss, Inc.