CODING POTENTIAL OF THE X-REGION OF HUMAN T-CELL LEUKEMIA LYMPHOTROPIC VIRUS TYPE-II/

Human T-cell leukemia/lymphotropic viruses type I and type II (HTLV-I and HTLV-II) are complex human retroviruses showing a similar genetic organization but substantially different biologic and pathogenic prope rties. As in other complex retroviruses, the 3' portion of the HTLV ge nome contains the peculiar ''X region'' comprising several partially o verlapping open reading frames (ORFs). To search for a possible basis for the pathogenic differences between the two viruses, a number of st udies have been carried out to analyze the coding potential of the X r egion of HTLV-I and, more recently, of HTLV-II. This review focuses on the coding potential or the HTLV-II X region and presents a compariso n with that of HTLV-I. Expression of different ORFs present in the X r egion may be accessed through two expression strategies: alternative s plicing and translation of more than one protein from the same mRNA. I nitial analyses of the X region proteins indicate that some differ sig nificantly from their HTLV-I homologues, thus providing possible clues to the understanding of the complex life cycle and pathogenicity of t he two viruses.