Basis of HTLV type 1 target site selection

Sequencing integration sites from >/=200 proviruses isolated from infected individuals revealed that HTLV-1 integration is not random at the level of the nucleotide sequence. The virus was found to integrate in A/T-rich regions with a weak consensus sequence at positions within and without the hexameric repeat generated during integration. These features were not associated with a preference for integration near active regions or repeat elements of the host chromosomes. However, about 6% of HTLV-1 proviruses were found to be integrated into transcription units, suggesting that in some cells, HTLV-1 integration may alter gene expression in vivo. Therefore, the target choice in vivo seems to be determined by local features rather than by the accessibility of specific regions. This led us subsequently to analyze the role of the DNA structure in HTLV-1 integration in vitro. Double-strand HTLV-1 or HIV-1 3' LTR extremities were used as substrates for in vitro strand transfer reactions using highly purified HTLV-1 and HIV-1 integrases (INs) expressed in Escherichia coli, and two synthetic naked 50-bp double-strand DNA molecules harboring different structures were used as targets. A fluorometric quantitative analysis of integration products was designed to assess the reaction efficiency for both target sequences. As suggested for HTLV-1 in vivo (present results), and, as previously described for other retroviruses in vitro, the structure of the target was found to greatly influence the site and the efficiency of integration. Both HIV-1 and HTLV-1 INs underwent the same target structural constraint, i.e., a strong preference for curved DNA. Altogether these results indicate that if most or all the regions of the genome appear to be accessible to HTLV-1 integration, local DNA curvature seems to confer a kinetic advantage for both in vitro and in vivo HTLV-1 integration.