THE RECOMBINANT PRODUCT OF THE CHRYPTOMONAS-PHI PLASTID GENE HLPA IS AN ARCHITECTURAL HU-LIKE PROTEIN THAT PROMOTES THE ASSEMBLY OF COMPLEXNUCLEOPROTEIN STRUCTURES

The HlpA protein which is encoded by the hlpA gene in the plastid geno me of the cryptomonad alga Chryptomonas Phi is structurally related to the non-sequence-specific DNA-binding and DNA-bending HU family of ch romatin-associated proteins. The expression of the HlpA protein comple ments the mutant phenotype of Bacillus subtilis cells impaired in the Hbsu protein (B. subtilis HU) as measured by the resistance of the cel ls to methylmethane sulphonate. To analyse the interactions of HlpA wi th DNA, we expressed the protein in Escherichia coli and purified it t o homogeneity. HlpA interacts preferentially with four-way junction DN A or DNA minicircles, when compared with Linear DNA, recognising DNA s tructure, HlpA and E, coli HU display comparable affinities far all ty pes 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 o f topoisomerase I, HlpA constrains negative superhelical turns in clos ed circular plasmid DNA. The HlpA protein mediates the joining of dist ant recombination sites into a complex nucleoprotein structure, as jud ged by beta-mediated site-specific recombination. The results presente d provide evidence that HlpA is a functional plastid equivalent of nuc lear and mitochondrial HMG1-like proteins and bacterial HU proteins.