Induction of the SOS regulon of Haemophilus influenzae does not affect phase variation rates at tetranucleotide or dinucleotide repeats

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Abstract
Haemophilus influenzaehas microsatellite repeat tracts in 5′ coding regions or promoters of several genes that are important for commensal and virulence behaviour. Changes in repeat number lead to switches in expression of these genes, a process referred to as phase variation. Hence, the virulence behaviour of this organism may be influenced by factors that alter the frequency of mutations in these repeat tracts. InEscherichia coli, induction of the SOS response destabilizes dinucleotide repeat tracts.H. influenzaeencodes a homologue of theE. coliSOS repressor, LexA. TheH. influenzaegenome sequence was screened for the presence of the minimal consensus LexA-binding sequence fromE. coli, CTG(N)10CAG, in order to identify genes with the potential to be SOS regulated. Twenty-five genes were identified that had LexA-binding sequences within 200 bp of the start codon. AnH. influenzaenon-inducible LexA mutant (lexANI) was generated by site-directed mutagenesis. This mutant showed increased sensitivity, compared with wild-type (WT) cells, to both UV irradiation and mitomycin C (mitC) treatment. Semi-quantitative RT-PCR studies confirmed thatH. influenzaemounts a LexA-regulated SOS response following DNA assault. Transcript levels oflexA,recA,recN,recX,ruvAandimpAwere increased in WT cells following DNA damage but not inlexANIcells. Induction of theH. influenzaeSOS response by UV irradiation or mitC treatment did not lead to any observable SOS-dependent changes in phase variation rates at either dinucleotide or tetranucleotide repeat tracts. Treatment with mitC caused a small increase in phase variation rates in both repeat tracts, independently of an SOS response. We suggest that the difference betweenH. influenzaeandE. coliwith regard to the effect of the SOS response on dinucleotide phase variation rates is due to the absence of any of the knowntrans-lesion synthesis DNA polymerases inH. influenzae.