The 5'-untranslated region (5'-UTR) of RNA of tobacco mosaic virus (TMV), called omega sequence, is known as an mRNA leader promoting efficient initiation of translation. The central part of the sequence consists of many CAA repeats, which were reported to be mainly responsible for the enhancing activity of the omega leader. In this work we synthesized the polyribonucleotides containing either the natural omega sequence or the regular (CAA)(n) sequence, and studied them using UV spectrophotometry and analytical ultracentrifugation methods. It was demonstrated that the polyribonucleotides manifest significant hypochromicity, cooperative melting of their structures upon heating, high melting temperature, and the sedimentation coefficients typical of compactly folded RNAs of this size. Thus, the omega leader and its core (CAA)(n) repeat sequence devoid of secondary structure of the Watson-Crick type seem to be well structured elements of mRNA. (c) 2007 Elsevier Inc. All rights reserved.