PHASE VARIATION OF HEMOGLOBIN UTILIZATION IN NEISSERIA-GONORRHOEAE

Most Neisseria gonorrhoeae isolates are unable to use human hemoglobin as the sole source of iron for growth (Hgb(-)), but a minor populatio n is able to do so (Hgb(+)). This minor population grows luxuriously o n hemoglobin, expresses two outer membrane proteins of 42 kDa (HpuA) a nd 89 kDa (HpuB), and binds hemoglobin under iron-stressed conditions. In addition to the previously reported HpuB, we identified and charac terized HpuA, which is encoded by the gene hpuA, located immediately u pstream of hpuB. Expression of both proteins was found to be controlle d at the translational level by frameshift mutations in a run of guani ne residues within the hpuA sequence encoding the mature HpuA protein. The ''on-phase'' hemoglobin-utilizing variants contained 10 G's, whil e the ''off-phase'' variants contained 9 G's. Insertional hpuB mutants of FA19 Hgb(+) and FA1090 Hgb(+) no longer expressed HpuB but still p roduced HpuA. A polar insertional mutation of the upstream hpuA gene i n FA1090 Hgb(+) eliminated production of both HpuA and HpuB, whereas a nonpolar insertional mutant expressed HpuB only. Insertional mutagene sis of either hpuA or hpuB or both substantially decreased the hemoglo bin binding ability of the FA1090 Hgb(+) variant and prevented growth on hemoglobin plates. Therefore, both HpuA and HpuB were required for the utilization of hemoglobin for growth.