Three mutants of the Escherichia coli hupA gene, encoding the HU-2 protein, were constructed by synthetic oligodeoxyribonucleotide-directed, site-specific mutagenesis on M13mp18 vectors. The resulting HupAN10, HupAN11 and HupAN12 proteins contained Thr59-->Lys, Gln64-->Lys and Asn53-->Arg substitutions, respectively. These amino acid (aa) changes increased the positive charge of the N-terminal half of the two-strand, antiparallel beta-ribbon of the arm structure, which is believed to be a domain for DNA binding. The three mutant proteins bound to DNA more tightly than wild-type HU-2, and their affinities for DNA increased in the order of HupAN10, HupAN11, HupAN12. The mutant proteins showed a slightly increased HU activity for supporting Mu phage development. A mutant HU-2 protein with increased basicity, but with an altered aa sequence in the arm region due to a frameshift mutation, was also constructed. This mutant protein showed a reduced affinity to DNA and was unable to support Mu growth, suggesting that a unique aa sequence of the arm domain, rather than mere basicity of this domain, is required for efficient binding to DNA.