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Hibiscus chlorotic ringspot virus p27 and Its Isoforms Affect Symptom Expression and Potentiate Virus Movement in Kenaf (Hibiscus cannabinus L.)

¹Department of Plant Pathology and ²State Key Laboratory of Agrobiotechnology, China Agricultural University, Beijing 100094, China; ³Temasek Life Sciences Laboratory, 1 Research Link, 117604, Singapore; ⁴Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Kent Ridge, 117543, Singapore

Hibiscus chlorotic ringspot virus (HCRSV), a member of the genus Carmovirus, encodes p27 (27-kDa protein) and two other in-frame isoforms (p25 and p22.5) that are coterminal at the carboxyl end. Only p27, which initiates at the ²⁵⁷⁰CUG codon, was detected in transfected kenaf (Hibiscus cannabinus L.) protoplasts through fusion to a Flag tag at either its N or C terminus. Subcellular localization of a p27-green fluorescent fusion protein in kenaf epidermal cells showed that it was localized to membrane structures close to cell walls. To study the functions of these proteins, a number of start codon mutants and premature translation termination mutants were constructed. Phenotypic differences were observed between the wild-type virus and these mutants during infection. Infectivity assays on plants indicated that p27 is a determinant of symptom severity. Without p25, appearance of symptoms on systemically infected kenaf leaves was delayed by 4 to 8 days. In a timecourse analysis, Western blot assays revealed that the delay corresponded to retardation in virus systemic movement, which suggested that p25 is probably involved in virus systemic movement. Mutations disrupting expression of p22.5 did not affect symptoms or virus movement.

Additional keywords: transmembrane domain , noncanonical start codon .