This research focuses on a cap-independent translation element (TE) we identified in the RNA of barley yellow dwarf virus (BYDV). Assisted by the TE, all messenger RNAs of BYDV, which include BYDV genomic RNA and subgenomic RNAs, are translated cap-independently. This allows uncapped, nonpolyadenylated BYDV RNAs to compete with capped and polyadenylated host mRNAs for the translation machinery. The core of the TE is a 105 nt sequence (TE105) located in the 3' UTR of BYDV genomic RNA. Due to its 3' position, at least two functions are necessary for TE105: 3 '--5' communication, and recruitment of translation machinery. TE105 also functions in the 5' UTR. In this setting, the 3'--5' communication function is unnecessary because of the 5' position of TE105. Using secondary structure probing and structure-directed mutagenesis, I discovered the cruciform secondary structure of TE105 and roles of domains within it. A seven-base loop sequence of stem-loop III in TE105 is involved in the 3'--5' communication because it is required only in the 3' UTR context. All other secondary structural elements, and a highly conserved sequence that may base pair directly to the 18S ribosomal RNA, are required for cap-independent translation in either context. Another sevenbase loop sequence identified in a 5' UTR stem-loop structure is also necessary for TE105 to function in the 3' UTR. RNA structural probing and functional analysis demonstrated that the loop sequences are base paired to each other, and that this pairing is necessary for the 3' --5' communication. This base-pairing is also essential for viral replication, probably through the translational regulation of the viral replicase gene. These results show that a closed-loop mRNA formed by RNA-RNA interaction between the UTRs is required for cap-independent translation of BYDV. This provides a new means of forming a closed-loop mRNA in the absence of a 5' cap and a poly(A) tail. The significance of this study towards the fundamental understanding of translation and possible practical applications of the TE sequence in transgenic plants are also discussed.