Intracistronic transcription termination in polysialyltransferase gene (siaD) affects phase variation in Neisseria meningitidis

Expression of serogroup B meningococcal capsular polysaccharide is subject to frequent phase variation. A reversible +1/-1 frameshift mutation within a poly(dC) repeat altering the reading frame of the polysialyltransferase g ene (siaD), thereby causing premature arrest of translation, is responsible for loss of capsule expression. After analysis of transcription of the sia D gene from an encapsulated strain and from two unencapsulated derivatives, we have found that the siaD mRNA in the unencapsulated strains is reduced in size as a result of premature transcription termination at a cryptic Rho -dependent site within the proximal region of the siaD cistron. Termination is sensitive to bicyclomycin, a natural inhibitor of Rho activity. Bicyclo mycin decreased the rates of capsule re-expression (off-on) without affecti ng the rates of loss of capsule expression (on-off). This finding suggested the existence of a novel mechanism linking transcription elongation termin ation and mutation frequency. A genetic system was therefore developed to m easure phase variation of siaD-ermC gene fusions in wild type and Rho-defec tive Escherichia coli strains. These studies demonstrated that in the Rho-d efective E. coli strain readthrough transcription of the mutated siaD gene caused a fourfold lower off-on phase variation rate than in the congenic Rh o(+) strain. Analysis of phase variation of siaD-ermC' gene fusions in a DN A mismatch-defective E. coli strain suggests that the effect of transcripti on on mutation rates required a functional mismatch repair system.