In Pseudomonas aeruginosa (PA), the main pathway for mucoid
conversion is mutagenesis of the mucA gene, frequently due to -1 bp
deletions in a simple sequence repeat (SSR) of 5 Gs (G5-SSR ). We 426
have recently observed that this mucA mutation is particularly
accentuated in Mismatch Repair System (MRS)-deficient cells
grown in vitro. Here, we evaluated the role of G5-SSR in 426
conversion to mucoidy under a MRS-deficient background. Thus,
mucA alleles were engineered with different contents of G:C SSRs,
and tested for their effect on the mucoid conversion frequency and
mucA mutational spectra in a mutS-deficient PA. Deletion of G5-
SSR severely reduced the emergence frequency of mucoid 426
variants, with no preferential site of mutagenesis within mucA.
Moreover, although mutagenesis in mucA was not totally removed,
this was no longer the main pathway for mucoid conversion,
suggesting that G5-SSR biased mutations towards mucA. 426
Mutagenesis in mucA was restored by the addition of a new SSR (C6-
SSR ), and even synergistically increased when G5-SSR and C6- 431 426
SSR were present simultaneously, with mutations being restricted 431
to -1 bp deletions within any of both G:C SSRs. These results
confirm a critical role for G5-SSR in mucA mutagenesis of MRS- 426
deficient cells, and shed light on the contribution of SSR-dependent
hypermutability in PA mucoid conversion.
