Phase variation is the high frequency, reversible switching of genetic elements, such that the gene product is either produced (said to be ON) or not produced (OFF). This phenomenon is typically caused by errors in DNA replication, when slippages occurs in nucleotide repeat tracts, and consequently alters promoter strength or shifts the reading frame of the gene. In cases where the phase-variable element is a regulator protein, phase variation is also applicable to the members of the regulon – which is consequently known as a phasevarion (phase-variable regulon).
Neisseria meningitidis strains can possess up to three independent, epigenetic, phase-variable regulators, encoded by the modA, modB and modD genes. Each of these genes possesses a number of allelic variants that differ in the DNA recognition domain sequence, and hence the corresponding Mod proteins regulate different phasevarions. The distribution and specific allele frequency of the mod genes varies in N. meningitidis sequence types and clonal complexes, however some associations between allele and pathogenic sequence types are seen. Most notably, the modD1 allele has been shown to specifically and significantly associate with hypervirulent, clonal complex 41/44 strains, but is absent from all screened carriage strains.
Our studies demonstrate that ModD1 is a functional adenine methyltransferase that methylates the sequence motif 5’-CCm6AGC-3’ (as identified by single molecule real time sequencing (SMRT) analysis). In addition, ModD1 regulates a phasevarion of over 20 genes, many of which are associated with colonisation and virulence of N. meningitidis. To further clarify the role of ModD1, a number of phenotypic assays have been conducted, including adherence and invasion of airway epithelial cells (HBE14 and Δ549 cells), and survival assays in human blood and sera. These studies will help elucidate the role that phase-variable regulators play in the virulence by N. meningitidis isolates.