The Internal Extra Sequence Regions in Satellite RNA TA-Tb Are Important for Suppressing RNA Accumulations of Cucumber Mosaic Virus to Attenuate the Virulence of the Helper Virus

Cucumber mosaic virus (CMV) infection is often associated with satellite RNA (satRNA), which can sometimes interfere with the replication and symptom expression of CMV. However, the mechanism underlying symptom attenuation has remained unclear. We previously discovered a larger type (than the usual type) of satellite RNA (satRNA TA-Tb) of CMV that reduced the symptom severity of CMV. Herein, we show that satRNA TA-Tb is associated with a reduction in CMV RNA accumulation, and particularly, a strong reduction of RNA4 accumulation at later stages of infection. Deletion analysis showed that the deletion of ten nucleotides of 5 ' and 3 ' termini, but not the internal sequence regions proximal to the 5 '- and 3 '-terminal regions, abolished satRNA TA-Tb replication. The alignment of satRNA TA-Tb with usual satRNA isolates showed four internal extra sequence regions (exR1-4) in satRNA TA-Tb. A satRNA TA-Tb mutant with deletion in the exR1 region retained the ability to attenuate CMV symptoms, whereas deletion of the exR2-4 regions abolished the attenuating effect of satRNA TA-Tb, but did not affect its replication. Overall, these results suggest that some short, internal extra sequence regions are dispensable for satRNA TA-Tb replication, but important for symptom attenuation function, supporting the possibility that the RNA structure of satRNA TA-Tb is important for its function in symptom attenuation.