Genomes of evolutionarily divergent members of the human T-cell leukemia virus family (HTLV-I and HTLV-II) are highly conserved, especially in pX.

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Abstract
Human T-cell leukemia virus (HTLV) is a family of related human T-lymphotropic retroviruses closely linked with certain human T-cell malignancies and associated with cases of acquired immunodeficiency syndrome (AIDS). HTLV was isolated and molecularly cloned from patients with both types of clinical disorders. Restriction endonuclease mapping and core and envelope protein analysis showed that at least 2 evolutionarily divergent viral subgroups exist, HTLV-I and HTLV-II. Previous studies failed to detect significant nucleotide sequence homology between HTLV-I and HTLV-II even though these different members of the HTLV family share certain biologic properties such as T-cell tropism and transformation. To further test these viruses for conserved regions in their genomes, hybridization was examined between HTLV-I and HTLVII by using Southern blotting and heteroduplex mapping at different melting points. These 2 techniques produced similar results, showing that HTLV-I and HTLV-II proviruses have strongly conserved nucleotide sequences in the pX region and lesser although still substantial homology in the long terminal repeat, gag, pol and env regions. These data provide experimental evidence that HTLV-II, like HTLV-I, contains pX sequences. Although the function of pX is unknown, its conservation in evolutionarily divergent human T-lymphotropic viruses implies a biologically important function. It is possible, but unproven, that pX could encode proteins involved in T-cell tropism, cell transformation immune suppression or other biologic actions characteristic of the HTLV family.