Variation in expression of the Haemophilus influenzae HMW adhesins: A prokaryotic system reminiscent of eukaryotes

Abstract

Expression of a number of eukaryotic genes is regulated by long stretches of tandem repeats located within the 5 untranslated region of the particular gene. In this study, we describe a regulatory system in Haemophilus influenzae with striking similarities to those found in eukaryotes. We show that expression of the HMW1 and HMW2 adhesins varies based on the number of 7-bp tandem repeats in the hmw1A and hmw2A promoters. The repeats lie between two separate transcription initiation sites and exert a repressive effect, such that increases in repeat number result in step-wise decreases in levels of specific mRNA and protein production and vice versa. The range of expression of HMW1 and HMW2 varies between very weak and very strong, with a series of gradations in between. Variation in the number of repeats in the hmw1A and hmw2A promoters occurs in individual colonies passaged in vitro, in an animal model of infection, and during natural infection in humans. This system of regulation is unique in prokaryotes and likely enhances the pathogenicity of the organism by increasing adaptive potential.In recent years, long stretches of tandem oligonucleotide repeats have been found increasingly in the upstream untranslated region of eukaryotic genes. In many cases, the number of repeats influences gene expression, reflecting the fact that transcriptional regulators often bind to the repeat region and regulate transcription (1). For example, the gene encoding human epidermal growth factor receptor is preceded by a series of TCC repeats that are recognized by a transcriptional activator. As these repeats are sequentially deleted, transcriptional activity and protein expression steadily decrease (2). Other examples in which upstream repeats function as transcriptional activating elements include the yeast Adr2p gene, the Drosophila hsp26 and actin 5C genes, the chicken ␣-2(I)collagen gene, and the rat neu gene (3-7). Less commonly, tandem repeats have a negative effect on downstream gene activity (8). Typically, the number of repeats undergoes highfrequency spontaneous variation via slipped-strand mispairing, serving as a source of quantitative genetic variation. Such quantitative variation is believed to confer an evolutionary advantage to a population, facilitating adaptation to novel ecological challenges.Nontypable Haemophilus influenzae is a Gram-negative bacterium that represents a common commensal organism in the human upper respiratory tract and an important cause of localized respiratory tract disease (8). The pathogenesis of disease begins with colonization of the upper respiratory mucosa, followed by contiguous spread to the middle ear, the sinuses, or the lungs, usually precipitated by respiratory viral infection or exacerbation of underlying allergic disease (9, 10). Similar to the situation with other bacterial pathogens, adherence to respiratory epithelium is a fundamental step in the process of colonization and is mediated by bacterial surface factors called adhesins. Among diverse strains of nontypabl...

Mapping of Binding Domains of Nontypeable Haemophilus influenzae HMW1 and HMW2 Adhesins

“…These proteins are phase variable (18,26), and switching from one Opa protein to another alters the binding and invasion characteristics of the organism (7,9,14,17,36). Similar to observations with the Opa proteins, we have recently demonstrated that both HMW1 and HMW2 are subject to phase variation, with potential influences of this variation on ultimate cellular interactions (6).…”

Evolutionary and Functional Relationships among the Nontypeable Haemophilus influenzae HMW Family of Adhesins

“…The variation is most marked in assays with HEp-2 cells examining NTHi derivatives expressing only HMW2, with levels ranging from ϳ10% (strain 5) to ϳ50% (strain 15). The differences in adherence were not due to variation in the quantity of protein, as examination of the promoter region of the hmwA genes in strains 12, 5, and 15 revealed a similar number of 7-bp repeats, which are known to influence the quantity of HMW protein produced (data not shown) (10). Differential glycosylation of the HMW adhesins by NTHi may provide a partial explanation for the variation in adherence patterns between strains and is currently under investigation.…”

Identification of New Genetic Regions More Prevalent in Nontypeable Haemophilus influenzae Otitis Media Strains than in Throat Strains

“…Our current model holds that natural selection and clonal expansion provide a diverse population of bacteria that can occupy various environmental niches, and phase variation provides a rapid response mechanism that allows the bacteria to adapt to quickly changing environments. Interestingly, two of the sPCR fragments found in this study to be more prevalent among otitis media strains represent genes that appear to be phase variable; sJPX163 represents hgpB, whose coding region contains multiple tetranucleotide (CCAA) repeats (31); and sJPX140 represents hmwA, whose upstream region contains a number of septanucleotide (ATCTTTC) repeats (11,52).…”