Phase Variation of Hemoglobin Utilization in Neisseria gonorrhoeae

Abstract

ABSTRACT Most Neisseria gonorrhoeae isolates are unable to use human hemoglobin as the sole source of iron for growth (Hgb − ), but a minor population is able to do so (Hgb + ). This minor population grows luxuriously on hemoglobin, expresses two outer membrane proteins of 42 kDa (HpuA) and 89 kDa (HpuB), and binds hemoglobin under iron-stressed conditions. In addition to the previously reported HpuB, we identified and characterized HpuA, which is encoded by the gene hpuA , located immediately upstream of hpuB . Expression of both proteins was found to be controlled at the translational level by frameshift mutations in a run of guanine residues within the hpuA sequence encoding the mature HpuA protein. The “on-phase” hemoglobin-utilizing variants contained 10 G’s, while the “off-phase” variants contained 9 G’s. Insertional hpuB mutants of FA19 Hgb + and FA1090 Hgb + no longer expressed HpuB but still produced HpuA. A polar insertional mutation of the upstream hpuA gene in FA1090 Hgb + eliminated production of both HpuA and HpuB, whereas a nonpolar insertional mutant expressed HpuB only. Insertional mutagenesis of either hpuA or hpuB or both substantially decreased the hemoglobin 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.