A recent study has shown that the reovirus lambda1 protein can unwind double-stranded nucleic acid molecules, a process energetically coupled to the hydrolysis of nucleoside 5'-triphosphates. In the present study, it was demonstrated that lambda1, expressed as a fusion protein with the Escherichia coli maltose-binding protein (MBP), possesses a non-specific affinity for various nucleic acids. The study also showed that the first ten amino acids of the protein are sufficient for binding while a deletion of the first five amino acids prevented the binding to nucleic acids. These data indicate that the basic charges located at the extreme amino-terminal portion of the protein constitute the major determinant responsible for nucleic acids binding. Finally, a quantitative gel retardation assay using purified solubilized MBP-lambda1 fusion protein confirmed the importance of the amino-terminal region and also revealed that the lambda1 protein possesses a higher affinity for single-stranded RNA than for double-stranded nucleic acids, a property shared by many helicases.