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The adenovirus tripartite leader is a 200-nucleotide-long 5' noncoding region which facilitates translation of viral mRNAs at late times after infection. The tripartite leader also confers the ability to initiate translation independent of the requirement for cap-binding protein complex or eIF-4F without any requirement for adenovirus gene products. To elucidate the manner by which the tripartite leader functions, the primary determinants of leader activity were investigated in vivo by testing a series of mutations expressed from transfected plasmids. The results of these experiments indicate that the tripartite leader does not promote internal ribosome binding, at least in a manner recently described for picornavirus mRNAs. In addition, despite an unusual arrangement of sequences complementary to the 3' end of 18S rRNA in the tripartite leader, we could find no evidence for involvement in its translation activity. Instead, our results are consistent with a model in which much of the first leader is maintained in an unstructured conformation which determines the ability of the tripartite leader to facilitate translation and bypass a normal requirement for eIF-4F activity. Several possible translation models are discussed, as well as the implications for translation of late viral mRNAs.