Lst is an outer membrane alpha-2,3-sialyltransferase (lst) that sialylates the lipooligosaccharide (LOS) of pathogenic Neisseria, facilitating their resistance to complement-mediated killing during infection. While it has been shown unequivocally that LOS sialylation is responsible for converting serum sensitive strains of N. gonorrhoeae (Ng) to serum resistance, the role of LOS sialylation in mediating serum resistance of N. meningitidis (Nm) is far less well understood; it may play a minor role in presence of capsule.

To further investigate the disparity of the role of LOS sialylation between Ng and Nm, we compared the sialyltransferase (stase) activity in Triton extracts of 25 Ng and Nm clinical isolates. Ng strains have on an average 3-4 fold more stase activity than of Nm strains. To examine the molecular mechanisms that explain the differential stase activity between the two pathogenic neisserial species, we conducted experiments to study the levels and stability of lst mRNA using Northern blot and real-time PCR analysis. lst mRNA was 5 ± 0.8 times more abundant and had a longer half-life in Ng strains than in Nm strains. Promoter mapping by primer extension analysis showed that the 3' terminus of a 105 base pair insertion element found exclusively in Nm strains acts as a promoter. We conclude that the differential expression of lst results from the differential transcription of lst and differential degradation of lst transcripts.