Diversity of tRNA Clusters in the Chloroviruses

Abstract

ABSTRACTViruses rely on their host’s translation machinery for the synthesis of their own proteins. Problems belie viral translation when the host has a codon usage bias (CUB) that is different from an infecting virus with differences in the GC content between the host/virus genome. Here, we evaluate the hypothesis that chloroviruses adapted to host CUB by acquisition and selection of tRNAs that at least partially favor their own CUB. The genomes of 41 chloroviruses comprising three clades of three different algal hosts have been sequenced, assembled and annotated. All 41 viruses not only encode tRNAs, but their tRNA genes are located in clusters. One tRNA gene was common to all three clades of chloroviruses, while differences were observed between clades and even within clades. By comparing the codon usage of one chlorovirus algal host, whose genome has been sequenced and annotated (67% GC content), to that of two of its viruses (40% GC content), we found that the viruses were able to at least partially overcome the host’s CUB by encoding tRNAs that recognize AU-rich codons. In addition, 39/41 chloroviruses encode a putative lysidine synthase, which alters the anticodon of tRNAmet that normally recognizes AUG to recognize the codon AUA, a codon for isoleucine. This is advantageous to the viruses because the AU-rich codon AUA is 12-13 times more common in the chloroviruses than their host. Evidence is presented that supports the concept that chlorovirus tRNA clusters were acquired prior to events that separated them into the three clades.IMPORTANCEChloroviruses are members of a group of giant viruses that infect freshwater green algae around the world. More than 40 chloroviruses have been sequenced and annotated. In order to propagate efficiently, chloroviruses with low GC content must overcome the high GC content and codon usage bias (CUB) of their hosts. We provide support for one mechanism by which viruses can overcome host CUB. Specifically, the chloroviruses examined herein encode tRNAs whose cognate codons are common in the viruses but not in the host. Virus-encoded tRNAs that recognize AU-rich codons enable more efficient protein synthesis, thus enhancing viral propagation. The tRNA genes are located in clusters and the original tRNA gene cluster was acquired by the most recent common ancestor of the four chlorovirus clades. Furthermore, we show some conservation among all clades, but also substantial variation between and within clades, demonstrating the dynamics of viral evolution.