Belozerskii Institute of Physicochemical Biology, M.V. Lomonosov Moscow State University, str. 40, 1, Leninskie gory, Moscow, 119991 Russia, е-mail nikitin@mail.bio.msu.ru Supported by Russian Scientific Foundation, a priority program for fundamental research (grant ¹ 14-24-00007) Received September 20, 2014 Abstract Potato virus X (Potexvirus) is an important pathogen of number of economically significant agricultural plants of the Solanaceae family. <...> Potato virus X (PVX) virions are flexible filamentous particles 515 nm in length and 13.5 nm in diameter with a helical structure. <...> PVX genome consists of a single-stranded RNA (6435 nucleotides) which is capped and polyadenylated. <...> The study of various stages of the infection process of plant viruses during infection, including the assembly of viral particles is of great practical interest. <...> Identification of these mechanisms can be the basis for developing new approaches in virus-free crop production. <...> Herein, the initial stages of potato virus X (PVX) virion assembly were examined on the example of virus-like particles (VLPs) produced by incubation of PVX RNA with PVX coat protein (CP) in vitro. <...> The formation of identical set of VLP with discrete size under different incubation conditions (ionic strength, pH) was shown. <...> However, the amount of VLP of a different size changes depending on incubation conditions. <...> Most efficient VLP similar to the native virion size assembly has been achieved in a buffer of low ionic strength at pH 8.5. <...> PVX RNA fragments from 800 to 5700 nt in length which were protected by coat protein within VLPs were isolated. <...> Their individual analysis confirmed that all of them represent the 5´-terminal fragments of the genomic PVX RNA of different lengths. <...> Thus, it was revealed that RNAs within VLP are genomic RNA 5´-terminal fragments of different lengths. <...> Nucleotide sequence analysis of RNA fragments isolated from VLP of different sizes showed that sites capable to form RNA hairpins were discovered near the RNA 3´-end. <...> Probably, CP could «melt» RNA hairpins more or less efficiently and overcome «stop-signals» during the viral particles formation, depending on the conditions of incubation with the RNA in vitro <...>