Eukaryotic translation initiation is an important step for gene expression. This process is stimulated by the interaction of 5' cap and 3' poly(A) tail mediated by trans-acting factors. Positive sense viral RNA can differ from the cellular mRNA by lacking either a 5' cap, or 3' poly(A) tail or both. Therefore, various strategies have evolved by the RNA viruses to achieve efficient translation. Barley yellow dwarf virus-PAV serotype (BYDV-PAV) lacks both a 5' cap and 3' poly(A) tail. I found that a sequence (called 3'TE for 3' translation enhancer) at the 3'UTR of barley yellow dwarf virus-PAV serotype (BYDV-PAV) genome stimulates translation of uncapped mRNA in the presence of 5'UTR of genomic RNA both in wheat germ extract and in oat protoplasts. The function of the 3'TE can be also supported by the 5'UTR of subgenomic RNA1. A four base duplication (GAUC) in the 3'TE region abolished the stimulatory effect of the 3'TE. There is no difference in mRNA stability between mRNAs bearing wildtype 3'TE and those bearing the defective 3'TE. Capping of the mRNA with the defective or without the 3'TE restored its translatability. Moreover, the four base duplication in the 3'TE eliminated virus infection. In trans, the 3'TE RNA can inhibit translation of both uncapped mRNA containing 3'TE and capped mRNA that lacks any BYDV-PAV sequences. This trans-inhibition of translation can be reversed by the addition of exogenous eIF-4F (eukaryotic translation initiation factor 4F). The importance of the 3' TE to stimulate translation of uncapped mRNA is also illustrated by the fact that the antisense RNA of 3'TE can inhibit the translation. Judged by the inhibitory effect of antisense RNA of the 3'TE on the translation of uncapped mRNA, it is found the 5' end of BYDV-PAV virion RNA is not capped. The 3'TE RNA mimics the function of the subgenomic RNA2 in translation inhibition in trans. The 3'TE RNA or subgenomic RNA2 of BYDV-PAV trans-inhibits translation of genomic RNA more significantly than they inhibit subgenomic RNA1. A model of the 3'TE function during BYDV-PAV replication is proposed, in which the 3'TE sequence of the subgenomic RNA2 acts as a regulatory RNA to inhibit translation of genomic RNA at the late stage of virus replication.