Dispensability of 3′ tRNA-like sequence for packaging cowpea chlorotic mottle virus genomic RNAs

The 3′ ends of three genomic RNAs (gRNAs) of cowpea chlorotic mottle virus (CCMV) terminate in a highly conserved tRNA-like structure (3′TLS). To examine the intrinsic role played the 3′TLS in packaging, the competence of each gRNA lacking the 3′ TLS (ΔTLS-gRNA) to interact with dissociated coat protein (CP) subunits and form virions was assayed in vitro. In contrast to the well established requirement for the participation of either viral 3′TLS or host-tRNAs in the assembly of RNA-containing virions in brome mosaic virus (BMV; Choi, Y, G., Dreher, T. W., Rao, A. L. N. 2002. tRNA elements mediate the assembly of an icosahedral RNA virus. Proc. Natl. Acad. Sci. 99, 655-660), CCMV CP does not require the presence of viral TLS in cis or in trans. Similar in vitro assembly assays showed that CCMV CP subunits also packaged BMV RNAs lacking 3′ TLS as well as two other non-bromoviral RNAs although with lesser efficiency. To characterize sequences of CCMV RNA3 (C3) required for packaging, a series deletions was engineered into C3 and their effect on virus assembly was examined. It was observed that, unlike BMV RNA3 whose packaging requires a bipartite signal (Choi, Y. G., Rao, A. L. N. 2003. Packaging of brome mosaic virus RNA3 is mediated through a bipartite signal. J. Virol. 77, 9750-9757), packaging of C3 is independent of either movement protein (MP) ORF or CP ORF or 3′ non-coding regions. Based on the differential prerequisites identified in this study for the assembly of BMV and CCMV, we hypothesize that the adaptive condition for movement in monocotyledonous host has made packaging a necessary co-requirement for BMV.