Characterization of a sialyltransferase-deficient mutant of Neisseria gonorrhoeae strain F62: instability of transposon Tn1545 Delta 3 in gonococci and evidence that multiple genetic loci are essential for lipooligosaccharide sialylation

Neisseria gonorrhoeae strain JB1 was previously shown to be defective in th e sialylation of lipoologosaccharide (LOS) by exogenous CMP-NANA. The LOS c omponents synthesized by the mutant now have been shown by mass spectrometr y to be similar to those in the parental strain, F62, and to include the 4. 5 kDa widely conserved lacto-N-neotetraose component that can be sialylated . The same two LOS components could be sialylated on the surface of the mut ant and parental strains. One major component was sialylatable after chemic al extraction of the LOS from either strain. These data confirm that the mu tant, JB1, retains the ability to synthesize the LOS target required for th e conversion by sialylation of serum-sensitive gonococci to serum resistanc e. A single base frame-shift mutation was found in the Ist gene from the mu tant, resulting in the replacement of the final 61 amino acids at the C-ter minus of the sialyltransferase by four residues. Seventeen independent clon es of the Ist gene were isolated from the parental strain, but none of them complemented the sialyltransferase defect of the mutant and no sialyltrans ferase activity expressed from the clones could be detected in Escherichia coli. Although the data suggest that the mutant might be defective in genes at more than one chromosomal locus and that multiple loci are essential fo r sialyltransferase synthesis and activity, the alternative possibility, th at DNA adjacent to the Ist gene encodes a product which is toxic to E. coli , cannot be excluded. The site of insertion of the transposon Tn1545-Delta 3 in strain JB1 was cloned and sequenced. The transposon is located in an i ntergenic region adjacent to genes for a putative ATP-dependent transport p rotein, but encoding no recognizable function relevant to LOS sialylation. Evidence that transposon Tn1545-Delta 3 is unstable in gonococci is present ed. (C) 1998 Academic Press.