Negative regulator of tomato mosaic virus (ToMV) multiplication, but has no effect on the multiplication of cucumber mosaic virus (CMV). Limits the spreading of the virus (PubMed:18762309). Isoform BTR1S: binds preferentially and directly to the 5'terminal region of ToMV genomic RNA, and affects the efficiency of translation rather than mRNA stability (PubMed:18762309). Expressed in leaves, stems and siliques. Not induced by ToMV infection. Higher accumulation of ToMV in infected leaves.