Klebsiella Phage vB_KleM-RaK2 — A Giant Singleton Virus of the Family Myoviridae

At 346 kbp in size, the genome of a jumbo bacteriophage vB_KleM-RaK2 (RaK2) is the largest Klebsiella infecting myovirus genome sequenced to date. In total, 272 out of 534 RaK2 ORFs lack detectable database homologues. Based on the similarity to biologically defined proteins and/or MS/MS analysis, 117 of RaK2 ORFs were given a functional annotation, including 28 RaK2 ORFs coding for structural proteins that have no reliable homologues to annotated structural proteins in other organisms. The electron micrographs revealed elaborate spike-like structures on the tail fibers of Rak2, suggesting that this phage is an atypical myovirus. While head and tail proteins of RaK2 are mostly myoviridae-related, the bioinformatics analysis indicate that tail fibers/spikes of this phage are formed from podovirus-like peptides predominantly. Overall, these results provide evidence that bacteriophage RaK2 differs profoundly from previously studied viruses of the Myoviridae family.

巨型噬菌体vB_KleM-RaK2（RaK2）的基因组大小为346千碱基对（kbp），是目前已测序的最大的侵染克雷伯氏菌的肌尾噬菌体基因组。在RaK2的534个开放阅读框（ORFs）中，总计272个未在数据库中检测到同源序列。基于与已生物学定义的蛋白质的相似性，以及串联质谱（MS/MS）分析结果，研究人员为117个RaK2开放阅读框赋予了功能注释，其中包括28个编码结构蛋白的开放阅读框——这些结构蛋白在其他生物的已注释结构蛋白中未找到可靠同源物。电子显微图像显示，RaK2的尾丝上存在复杂的棘突状结构，这表明该噬菌体属于非典型肌尾噬菌体。尽管RaK2的头部和尾部蛋白大多与肌尾噬菌体科（Myoviridae）相关，但生物信息学分析显示，该噬菌体的尾丝/棘突主要由短尾噬菌体科（Podoviridae）样肽构成。总体而言，这些结果表明，噬菌体RaK2与此前已研究的肌尾噬菌体科（Myoviridae）病毒存在显著差异。