Biochemical and genetic data on retroviral nucleocapsid (NC) proteins have shown that this viral protein exhibits nucleic acid annealing and strand transfer activities and is required for the formation of infectious viral particles. However, the DNA binding properties of the NC protein of the human T-cell leukemia virus type-I (HTLV-I) has not been extensively studied. In this work we characterize the DNA binding ability of the zinc-bound and zinc-free forms of the p15 NC of HTLV-I. We found that only the zinc-bound form of the p15 NC binds single-stranded and double-stranded DNA fragments, but both forms of the p15 NC protein bind and unwind supercoiled DNA. The unwinding activity of the zinc-bound form was 3-fold higher than that observed with the zinc-free form of the protein. Interestingly, eukaryotic DNA topoisomerase antagonists inhibited this unwinding activity. In addition, we showed the formation of NC protein-DNA cleavable complex, which is the result of a presumably covalent bond formed between the protein and the phosphate moiety of the DNA backbone. Moreover, the presence of the p15 NC in the reverse transcription assay significantly increased the activity of the HTLV-I reverse transcriptase. These results demonstrate new DNA binding properties of the p15 NC protein and shed light on the possibility of a novel physiological function for the HTLV-I NC protein in the viral life cycle.

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