Differential expression and regulation of lipooligosaccharide specific sialyltransferase in pathogenic Neisseria

Transfer of sialic acid onto the lipooligosaccharide (LOS) of Neisseria gonorrhoeae (Ng) and Neisseria meningitides (Nm), the two pathogenic species of neisseria, is catalyzed by the outer membrane enzyme [alpha]-2, 3-sialyltransferase (Lst). Sialylated LOS affects different facets of neisserial pathogenesis and is considered a key virulence factor. LOS sialylation interferes with neisserial (1) entry into epithelial cells, (2) killing by neutrophils, (3) killing by complement and (4) recognition by bactericidal antibodies against outer membrane structures. Very little is known about the gene regulation in neisseria and that of neisserial sialyltransferase (stase). Studies about distribution of stase activity in 16 random isolates of Ng and Nm revealed that Ng isolates contained varying amounts of stase activity, but, on average, 2.2-fold more stase activity than extracts of Nm isolates, representing seven serogroups and non-groupable strains (Mann-Whitney U test, p < 0.001). Ng isolates also express more lst transcript and Lst protein than most Nm isolates. Thus constitutive stase expression is higher in Ng isolates compared to Nm isolates. Promoter mapping studies revealed that Ng and Nm use different promoters to transcribe lst. In Ng a strong sigma 70 promoter 80 bp upstream of the translational start site is used to transcribe lst, whereas this promoter present in meningococci lst upstream region is inactive. In Nm a weak sigma 70 promoter at the 3' terminus of an 105 bp CREE insertion sequence element 99 bp upstream of the translational start site is used to transcribe lst. The differential constitutive stase expression between Ng and Nm is due, at least in part, to use of different promoters which have different strengths. Another factor that contributes to differential lst expression in Ng and Nm is differential half-life of the lst transcript. Ng F62 lst mRNA has a longer half-life (t1/2 = 90 to 120 sec) than Nm MC58 (t1/2 = 40 to 45 sec). Thus constitutive stase expression in pathogenic neisseria is affected by CREE insertion and differential half-life of lst transcript. Sequencing the Lst structural gene revealed many point mutations in isolates of Ng and Nm that could affect stase activity in those isolates. We also provide evidence for the existence of neisserial factors that affect stase activity. Besides regulation of constitutive expression, stase is regulated by environmental cues. Ng F62 downregulates expression of lst upon contact with cervical epithelial (ME-180) cells. In conclusion, the regulation of stase expression among and between pathogenic neisseria occurs by (1) differential gene transcription, (2) differential mRNA half life and (3) by sensing the environment. The presence of multiple gene regulatory mechanisms underscores the importance of this enzyme in neisserial pathogenesis.