The long-lasting enigma of polycytidine (polyC) tract

Abstract

Long polycytidine (polyC) tracts varying in length from 50 to 400 nucleotides were first described in the 5′-noncoding region (NCR) of genomes of picornaviruses belonging to the Cardio- and Aphthovirus genera over 50 years ago, but the molecular basis of their function is still unknown. Truncation or complete deletion of the polyC tracts in picornaviruses compromises virulence and pathogenicity but do not affect replicative fitness in vitro, suggesting a role as “viral security” RNA element. The evidence available suggests that the presence of a long polyC tract is required for replication in immune cells, which impacts viral distribution and targeting, and, consequently, pathogenic progression. Viral attenuation achieved by reduction of the polyC tract length has been successfully used for vaccine strategies. Further elucidation of the role of the polyC tract in viral replication cycle and its connection with replication in immune cells has the potential to expand the arsenal of tools in the fight against cancer in oncolytic virotherapy (OV). Here, we review the published data on the biological significance and mechanisms of action of the polyC tract in viral pathogenesis in Cardio- and Aphthoviruses.

Engineering U1-Based Tetracycline-Inducible Riboswitches to Control Gene Expression in Mammals

“…Equivalent constructs were made with rsU1inON as controls. We used as insulator a string of 19 cytosines (19C), which has been described as linear and to function as an insulator for neighboring RNA structures . The 19C were cloned upstream (19C+rsU1inON and 19C+rsU1inC) or downstream (rsU1inON+19C and rsU1inC+19C) of the rsU1in sequences.…”

“…We used as insulator a string of 19 cytosines (19C), which has been described as linear and to function as an insulator for neighboring RNA structures. 35 The 19C were cloned upstream (19C+rsU1inON and 19C+rsU1inC) or downstream (rsU1i-nON+19C and rsU1inC+19C) of the rsU1in sequences. In addition, we tested the effect of a string of 19 guanines (19G) downstream along with 19C upstream of the rsU1in sequences (19C+rsU1inON+19G and 19C+rsU1inC+19G), which should form a strong stem that isolates the riboswitch.…”

Polycytidine tract deletion from microRNA-detargeted oncolytic Mengovirus optimizes the therapeutic index in a murine multiple myeloma model

“…However, all Mengovirus isolates analyzed to date have been reported to have polyC spacers in the 50–60-nt range, which indicates that some limitations unique to Mengovirus may need to be taken into consideration. 43 These observations suggest that the highly compact and efficient organization of the picornavirus genome may impose severe constraints that disallow even small variations in the RNA structural landscape. In consequence, the positioning and precise sequence of even small insertions in the Mengovirus genome can have a significant impact on viral fitness, as observed in the current study.…”

“…Accurately measuring the length of the polyC tract has proven to be a significant technical challenge due to the repetitive nature of the sequence. 43 PolyC deletion may therefore simplify the characterization of a manufactured product for clinical application because it eliminates the need to confirm that the tract length is below the threshold associated with pathogenicity. Also, as reported here, the polyC-deleted virus can be conveniently produced at 37°C.…”

“…Certain viral species in the Cardiovirus and Aphthovirus genera of the Picornaviridae family contain a polycytidine (polyC) tract 50 to 250 nucleotides long in the 5′ NCR. 43 In Mengovirus, the polyC tract, which occupies a putatively unstructured spacer region between a pseudoknot (pk) and a hairpin structure named stem-loop D, is 55 nucleotides long and contains a single uridine discontinuity, with sequence C 44 UC 10. 44 Although the exact function and molecular mechanism by which the polyC tract impacts the virus replication cycle are still to be elucidated for each viral species, several studies have established that the polyC length is a major Mengovirus pathogenicity determinant and that reducing its length proportionally decreases the virulence of the virus in vivo without destroying its replication competence.…”