FIGURE 6.

Schematic representation showing two distinct bacterial heme-sensing systems, one extracellular and one intracellular, that regulate heme homeostasis by HrtBA-mediated efflux. Left panel, extracellular heme-sensing induces the two-component HssSR system. Once stimulated, HssR activates hrtBA expression, as reported (25). Right panel, in L. lactis, heme is taken up by fhuDBA gene products (green ovals) (35).⁴ Internalized heme binds to available HrtR protein to relieve repression of the hrtRBA operon. Activation of hrtBA results in heme efflux. Red asterisk, heme. Blue lightning represents sensor activation.