H‐DNA formation by the coding repeat elements of neisserial opa genes

Summary The coding repeat region of opa genes from Neisseria gonorrhoeae and Neisseria meningitidis determines the expression state of their respective genes through high‐frequency addition or deletion of pentanucleotide coding repeat units (CRs; CTTCT). In vitro analyses of cloned opa gene CR regions using single‐strand specific nucleases, oligonucleotide protection experiments, and modifications of non‐B‐DNA residues indicate that the regions form structures resembling H‐DNA under acidic conditions in the presence of negative supercoiling. The purine/pyrimidine strand bias and H‐palindromic nature of the repeat region are consistent with sequence requirements for H‐DNA formation. Sequences flanking the repeat elements are required to form the H‐DNA structure in vitro as judged by the pattern of exposed non‐B‐DNA residues in CR sequences synthesized as oligonucleotides to form β‐galactosidase::CR translational fusions. The fusions phase vary by addition and deletion of CR elements and the rate of phase variation increases upon induction of the fusion genes. The opa gene CR region is the first reported bacterial H‐DNA structure and is unique in that it ties within the coding sequence for the gene.