A NOVEL CHROMATIN-FORMING HISTONE H1 HOMOLOG IS ENCODED BY A DISPENSABLE AND GROWTH-REGULATED GENE IN BORDETELLA-PERTUSSIS

We report the identification of a protein homologous to a histone H1 i n Bordetella pertussis. The B. pertussis histone homologue, BpH1, vari es in size in different strains from 182 to 206 amino acids. The varia bility of the size of the protein is due to gene variability by insert ion or deletion of DNA modules. Insertion of a kanamycin cassette into the bpH1 gene generates a BpH1 null mutant with phenotypic properties and growth rate similar to those of the wild-type strain, showing tha t this gene is dispensable. In vitro, the BpH1 protein prevents chromo somal DNA degradation from DNase I and constrains supercoiled DNA. Tra nscription of the bpH1 gene is activated during exponential growth of the bacteria, whereas it is repressed during the stationary phase of g rowth. It is proposed that BpH1 plays a role in chromatin formation an d condensation during DNA replication and that repression of transcrip tion depends upon a reduced rate of DNA replication.