Expression of the Moraxella catarrhalis UspA1 protein undergoes phase variation and is regulated at the transcriptional level.

Moraxella (Branhamella) catarrhalis is an unencapsulated gram-negative bacterium that can cause both upper and lower respiratory tract infections (14, 33). It has been estimated that M. catarrhalis causes approximately 20% of cases of acute bacterial otitis media in infants and young children (5) and is associated with nearly 30% of infectious exacerbations of chronic obstructive pulmonary disease in adults (17). The significant morbidity associated with M. catarrhalis infections as well as the substantial health care costs of these infections have prompted recent interest in the development of an M. catarrhalis vaccine (37). Proteins present in or closely associated with the outer membrane of M. catarrhalis strains obtained from diverse geographic and clinical sources display highly similar patterns when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (4) and have received the most attention as potential vaccine candidates. Several of these cell surface-exposed proteins have been characterized in some detail, including UspA1, UspA2 (HMWP), and UspA2H (24, 26, 32); OMP CD (21, 34); the iron-regulated CopB protein (3, 8); the LbpA and LbpB proteins (6); and the TbpA and TbpB proteins (7, 28, 35). Little is known about the regulation of expression of M. catarrhalis outer membrane proteins. Campagnari et al. (8) were the first to show that the availability of iron in the growth medium affected expression of several M. catarrhalis outer membrane proteins. A spontaneous mutant of M. catarrhalis that lacked the ability to express several different outer membrane antigens was described by Murphy and coworkers (25). In addition, it was reported that one strain of M. catarrhalis could give rise to variants that expressed a truncated UspA2 protein (K. R. VanDerMeid, S. M. Baker, and J. C. McMichael, Abstr. 99th Gen. Meet. Am. Soc. Microbiol. 1999, abstr. D/B-289, p. 256, 1999). Most recently, it was reported that the 200-kDa surface protein of this organism, which may be involved in hemagglutination (15), underwent phase-variable expression that involved apparent slipped-strand mispairing in a homopolymeric nucleotide repeat located within the open reading frame (ORF) encoding this protein. (K. Sasaki, L. Myers, S. M. Loosmore, and M. H. Klein, Abstr. 99th Gen. Meet. Am. Soc. Microbiol., 1999, abstr. B/D-306, p. 89, 1999). The UspA1 surface protein of M. catarrhalis is synthesized as an 80- to 90-kDa monomer that forms very large aggregates or complexes that are relatively resistant to heating in the presence of SDS (12, 32). This protein also has been shown to mediate attachment of this bacterium to Chang conjunctival epithelial cells in vitro (26). In the present study, expression of the M. catarrhalis UspA1 protein was found to exhibit phase variation. Nucleotide sequence analysis indicated that this phenotypic switch could be correlated with changes in the length of a homopolymeric nucleotide [poly(G)] tract located upstream of the uspA1 ORF. Primer extension, RNA slot blot, and Northern hybridization experiments revealed that UspA1 expression was regulated at the level of transcription. Cloning and expression of uspA1 genes in Haemophilus influenzae revealed that the changes in the length of the poly(G) tract were sufficient to account for the phase-variable expression of UspA1.