The HlpA protein which is encoded by the hlpA gene in the plastid genome of the cryptomonad alga Chryptomonas phi is structurally related to the non-sequence-specific DNA-binding and DNA-bending HU family of chromatin-associated proteins. The expression of the HlpA protein complements the mutant phenotype of Bacillus subtilis cells impaired in the Hbsu protein (B. subtilis HU), as measured by the resistance of the cells to methylmethane sulphonate. To analyse the interactions of HlpA with DNA, we expressed the protein in Escherichia coli and purified it to homogeneity. HlpA interacts preferentially with four-way junction DNA or DNA minicircles, when compared with linear DNA, recognising DNA structure. HlpA and E. coli HU display comparable affinities for all types of DNA tested; however, HlpA exhibits a stronger tendency to self-associate in the presence of DNA. Accordingly, HlpA oligomerises more readily than HU in protein crosslinking experiments. In the presence of topoisomerase I, HlpA constrains negative superhelical turns in closed circular plasmid DNA. The HlpA protein mediates the joining of distant recombination sites into a complex nucleoprotein structure, as judged by beta-mediated site-specific recombination. The results presented provide evidence that HlpA is a functional plastid equivalent of nuclear and mitochondrial HMG1-like proteins and bacterial HU proteins.