Concentration-dependent organization of DNA by the dinoflagellate histone-like protein HCc3

The liquid crystalline chromosomes of dinoflagellates are the alternative to the nucleosome-based organization of chromosomes in the eukaryotes. These nucleosome-less chromosomes have to devise novel ways to maintain active parts of the genome. The dinoflagellate histone-like protein HCc3 has significant sequence identity with the bacterial DNA-binding protein HU. HCc3 also has a secondary structure resembling HU in silico. We have examined HCc3 in its recombinant form. Experiments on DNA-cellulose revealed its DNA-binding activity is on the C-terminal domain. The N-terminal domain is responsible for intermolecular oligomerization as demonstrated by cross-linking studies. However, HCc3 could not complement Escherichia coli HU-deficient mutants, suggesting functional differences. In ligation assays, HCc3-induced DNA concatenation but not ring closure as the DNA-bending HU does. The basic HCc3 was an efficient DNA condensing agent, but it did not behave like an ordinary polycationic compound. HCc3 also induced specific structures with DNA in a concentration-dependent manner, as demonstrated by atomic force microscopy (AFM). At moderate concentration of HCc3, DNA bridging and bundling were observed; at high concentrations, the complexes were even more condensed. These results are consistent with a biophysical role for HCc3 in maintaining extended DNA loops at the periphery of liquid crystalline chromosomes.