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The UspA1 protein of Moraxella catarrhalis has been shown to function as an adhesin that mediates adherence to human epithelial cell lines in vitro (E. R. Lafontaine, L. D. Cope, C. Aebi, J. L. Latimer, G. H. McCracken, Jr., and E. J. Hansen, J. Bacteriol. 182:1364-1373, 2000). In the present study, cell lysates prepared from individual colonies of several M. catarrhalis wild-type strains were analyzed by Western blot analysis using monoclonal antibodies (MAbs) specific for the UspA1 protein. Expression of UspA1 was shown to exhibit phase variation that was correlated with both adherence ability in vitro and the number of guanine (G) residues contained within a homopolymeric [poly(G)]tract located upstream of the uspA1 open reading frame (ORF). Nucleotide sequence analysis revealed that isolates expressing relatively high levels of UspA1 had 10 G residues in their uspA1 poly(G)tracts, whereas isolates that expressed much lower levels of UspA1 had 9 G residues. This poly(G) tract was located 30 nucleotides (nt) upstream of the uspA1 ORF and 168 nt downstream of the uspA1 transcriptional start site. Primer extension experiments, RNA slot blot analysis, and cat reporter constructs were used to demonstrate that M. catarrhalis isolates with 10 G residues in their uspA1 poly(G) tracts expressed two-to threefold more uspA1 mRNA than did isolates which had 9 G residues in their poly(G)tracts. Northern hybridization analysis revealed that an intact uspA1 mRNA was readily detectable in RNA from M. catarrhalis isolates that had 10 G residues in their uspA1 poly(G) tracts, whereas no full-length uspA1 mRNA was observed in isolates whose poly(G)tracts contained 9 G residues. M. catarrhalis strain O35E uspA1 genes that contained wild-type and mutated poly(G) tracts were expressed in Haemophilus influenzae to demonstrate that the length and composition of the poly(G)tract affected expression of UspA1.

Expression of the Moraxella catarrhalis UspA1 Protein Undergoes Phase Variation and Is Regulated at the Transcriptional Level