The opa multigene family of Neisseria gonorrhoeae encodes 11 related o
uter-membrane proteins which phase vary in vitro and in vivo. Illegiti
mate recombination within direct pentameric DNA repeats, encoding the
signal-peptide region of pre-Opas, leads to switches in expression sta
tes. Despite the conserved nature of the variation mechanism, specific
genes are expressed at high frequencies in the transition from Opa- t
o Opa+. The genes which are expressed at elevated frequencies differ f
rom the rest of the family with respect to promoter structure, based o
n sequence comparisons between the cpa genes of strain MS11mk. We have
analysed transcription of the cpa gene family of N. gonorrhoeae MS11m
k, focussing on the different promoters found among the 11 genes to de
termine whether increased levels of expression are associated with inc
reased phase-variation rates. Primer extension and Northern blotting w
as used to assess the levels of transcription of three representative
opa genes (opaA, B and C) in 'on' and 'off' states. Full-length opa mR
NA was detected primarily in strains expressing the homologous gene. T
runcated opa mRNA was constitutively expressed from all cpa genes rega
rdless of their expression state. Quantitative comparisons in N. gonor
rhoeae were complicated by the simultaneous expression of all 11 genes
and the cross-reactivity of mRNA probes. Expression levels from the i
ndividual promoters were therefore assessed by creating transcriptiona
l and translational lacZ fusions to each of the representative cpa pro
moters which lacked the DNA repeats responsible for variation. The exp
ression levels were compared to the phase-variation rates of translati
onal opa::phoA fusions containing the same promoters in addition to th
e corresponding coding repeat regions. A strong correlation was found
between expression levels from the different promoters and the variati
on rates at which 'on' variants appeared from an 'off' population (i.e
. opaA > opaB > opaC). These results provide an explanation for the fa
voured expression of specific Opa proteins and indicate that expressio
n of cpa genes may be regulated at the level of transcription.