A feature of <em>Haemophilus influenzae</em> genomes is the presence of several loci containing tracts of six or more identical tetranucleotide repeat units. These repeat tracts are unstable and mediate high frequency, reversible alterations in the expression of surface antigens. This process, termed phase variation (PV), enables <em>H.influenzae</em> to rapidly adapt to fluctuations in the host environment. Perturbation of lagging strand DNA synthesis is known to destabilize simple sequence repeats in yeast and <em>Escherichia coli</em>. By using a chromosomally located reporter construct, we demonstrated that the mutation of an <em>H.influenzae rnhA</em> (encoding RnaseHI) homologue increases the mutation rates of tetranucleotide repeats ∼3-fold. Additionally, deletion of the Klenow domain of DNA polymerase I (PolI) resulted in a ∼35-fold increase in tetranucleotide repeat-mediated PV rates. Deletion of the PolI 5′>3′ exonuclease domain appears to be lethal. The phenotypes of these mutants suggest that delayed or mutagenic Okazaki fragment processing destabilizes <em>H.influenzae</em> tetranucleotide repeat tracts.© 2005, Christopher D. Bayliss, Wendy A. Sweetman and E. Richard Moxon. Nucleic Acids Research, Vol. 33 No. 1 © Oxford University Press 2005; all rights reserved. The online version of this article has been published under an open access model. Users are entitled to use, reproduce, disseminate, or display the open access version of this article for non-commercial purposes provided that: the original authorship is properly and fully attributed; the Journal and Oxford University Press are attributed as the original place of publication with the correct citation details given; if an article is subsequently reproduced or disseminated not in its entirety but only in part or as a derivative work this must be clearly indicated. For commercial re-use permissions, please contact email@example.com
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