Abstract
Recently, we hypothesized that silencing suppression activity gained by a viral replicative helicase led to the emergence of the second helicase possessing activity of the viral silencing suppressor and/or movement protein (MP). Our hypothesis accounted for the evolutionary origin of the specialized ‘triple gene block’ (TGB) in plant virus genomes encoding the MPs TGB1, TGB2, and TGB3 required for viral cell-to-cell transport through plasmodesmata. Here, we used public transcriptome databases to identify previously unrecognized viruses. The analysis of novel viral genomes further supported the previously proposed scenario of TGB origin and evolution, which included the following steps. First, the accessory helicase gene could have been acquired by horizontal gene transfer (HGT) presumably occured independently in different virus groups. Second, the TGB2 gene evolved by HGT or autonomization of the C-terminal transmembrane domain found in at least one TGB1 helicase. Third, the TGB3 gene has most likely emerged in the genomic block consisting of the TGB1 and TGB2 genes.
Highlights
For replicative RNA/DNA synthesis, plant viruses encode enzymes belonging to one of three helicase superfamilies (SFs), SF-I, SF-II, and SF-III (Koonin and Dolja, 1993; Iyer et al, 2004; Hickman and Dyda, 2005; Jankowsky and Fairman, 2007; Byrd and Raney, 2012; Gilhooly et al, 2013)
triple gene block (TGB) is a module consisting of three genes termed TGB1, TGB2, and TGB3, which encode movement proteins (MPs) necessary for viral cell-tocell transport
Triple gene block evolution view that the TGB1 protein can be regarded as an “accessory” helicase evolved after duplication of a replicative RNA helicase (Koonin and Dolja, 1993), we hypothesized that gaining new silencing suppression function by a replicative RNA helicase could precede subsequent helicase domain duplication concomitant with its autonomization, which can occur both in cis or in trans
Summary
For replicative RNA/DNA synthesis, plant viruses encode enzymes belonging to one of three helicase superfamilies (SFs), SF-I, SF-II, and SF-III (Koonin and Dolja, 1993; Iyer et al, 2004; Hickman and Dyda, 2005; Jankowsky and Fairman, 2007; Byrd and Raney, 2012; Gilhooly et al, 2013). Triple gene block evolution view that the TGB1 protein can be regarded as an “accessory” helicase evolved after duplication of a replicative RNA helicase (Koonin and Dolja, 1993), we hypothesized that gaining new silencing suppression function by a replicative RNA helicase could precede subsequent helicase domain duplication concomitant with its autonomization, which can occur both in cis (in the context of the same viral genome) or in trans (implying transfer to a foreign virus genome) These events may result in evolving specialized second helicase possessing the activity of viral silencing suppressor (VSR), or, taking into account a tight link between viral movement and silencing suppression, both suppression and movement functions (Burgyán and Havelda, 2011; Morozov and Solovyev, 2012; Pumplin and Voinnet, 2013). In an attempt to further resolve potential evolutionary relations between distant TGB-containing viruses, we performed new comparative sequence analyses of “accessory” helicases encoded in previously sequenced plant virus genomes, including NVMV and HGSV, and VLRAs found in public databases, the 1000 Plants (1KP) project (Matasci et al, 2014)
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