A planarian nidovirus expands the limits of RNA genome size

Amir Saberi; Anastasia A. Gulyaeva; John L. Brubacher; Phillip A. Newmark; Alexander E. Gorbalenya

RNA viruses may uniquely illuminate the evolutionary constraints on RNA genome size, whether or not they descended directly from primitive RNA-based entities (6) (7) (8) . The average-sized RNA virus genome is around 10 kb, due to low-fidelity RNA replication without proofreading (9) . However, positive-stranded RNA genomes of viruses of the order Nidovirales, which includes pathogenic coronaviruses, range from 12.7 to 33.5 kb, the largest known RNA genome (10, 11) (Fig. 1A , Table S1 ). These viruses may have evolved genomes larger than 20 kb after acquiring a proofreading exoribonuclease (ExoN) that improved replication fidelity (12) (13) (14) (15) (16) (17) . Despite this notable innovation, over the last 20 years of virus discovery, including unbiased metagenomics of invertebrate and vertebrate RNA viruses (18, 19) , the largest-known RNA viral genome has only increased ~10% in sizea mere fraction of the nearly ten-fold increase observed for DNA viruses (20) (21) (22) (Fig. 1A) . Apparently, genome sizes of large nidoviruses are tightly constrained by the interplay of genetic and hostspecific factors, with the size of open reading frame 1b (ORF1b), encoding key replicative enzymes, emerging as a major barrier for further genome expansion (23) . Hence, ~34 kb is considered the de facto limit of RNA virus genome size (18) .