Summary: The pili expressed by all isolates of react with two monoclonal antibodies, SM1 and SM2. In contrast, although many isolates of also express pili (class I) which react with antibodies SM1 and SM2, a proportion express pili (class II) which fail to react. In order to define the epitopes recognized by these antibodies, a series of overlapping peptides corresponding to the amino acid sequence of conserved regions of gonococcal pili have been synthesized. The minimum epitope recognized by antibody SM1 was found to comprise a linear peptide EYYLN, corresponding to residues 49-53 of mature pilin. In contrast, antibody SM2 reacted with a number of peptides from around the cysteine residue (Cys 1) at position 120, suggesting that an extended region may contribute to a conformational epitope recognized by this antibody in the native protein. The identification of the two epitopes defines structural differences between the classes of pili expessed by meningococci. In order to determine the distribution of pilin gene sequences in we used as hybridization probes an oligonucleotide (PS1) with the sequence 5′-GAGTATTACCTGAATCA-3′ which spans the coding region for the SM1 epitope, and a fragment of the 3′ end of the gonococcal gene which contains conserved sequences flanking the two Cys codons and encodes the SM2 epitope. All strains of and tested, regardless of piliation phenotype, harboured DNA sequences homologous to those encoding the carboxy-terminus of meningococcal class I pilin. Furthermore, all gonococci and all meningocococci producing class I pili hybridized with oligonucleotide probe PS1. Non-reverting non-piliated derivatives of previously class I pilus-producing strains showed reduced hybridization signals with this probe, but nevertheless retained sequences homologous to the coding sequence for the SM1 epitope. However, meningococci producing class II pili could be divided into two groups on the basis of their reaction with the PS1 probe: half the strains tested failed to react, which is consistent with our previous analysis of silent class I pilin sequences; the remainder reacted (relatively weakly) with the probe, suggesting that the silent sequences in these strains extend further towards the 5′ end of the pilin gene than in strains studied previously. Some strains of reacted weakly with both types of probe but failed to produce SM1-reactive pili. In contrast,


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