We have previously described HpuAB, a two-component receptor that mediates binding to haemoglobin (Hb), haemoglobin-haptoglobin (Hb-Hp) and apo-haptoglobin (Hp). In this communication, we constructed non-polar mutations in the hpuA and hpuB loci to examine the individual roles of HpuA and HpuB. Our results indicate that both HpuA and HpuB are required for the acquisition of Fe from Hb and Hb-Hp. We isolated Hb utilization-positive (Hb+) variants of our Hb utilization-negative (Hb-) hpu mutants at a frequency of 10(-3) and demonstrated that the Hb+ phenotype resulted from the expression of a second Hb receptor, HmbR. Expression of HmbR in DNM2 was found to be controlled by translational frameshifting involving a polyguanine (G) tract located within the hmbR locus. The hpuA locus also contains a poly(G) tract, which suggested that meningococci could phase vary each Hb receptor independently by slip-strand mispairing in the poly(G) tracts found in hpuA and hmbR. Thus, we isolated a naturally occurring Hb- variant of DNM2, designated DNM2 Hb-, which did not express either HpuAB or HmbR. Hb+ variants of DNM2Hb- were selected and examined for HpuAB and HmbR expression. In each instance, acquisition of HpuAB or HmbR expression was correlated with phase variation in the poly(G) tract of each Hb receptor.