FIG. 3.

(A) Induction of the ygf operon by heme. The expression of a chromosomal ygf lacLM fusion in strain MG1363 was monitored under static and aerobic growth without or with 100 μM heme. The β-galactosidase activity (bars, in Miller units, based on the cell OD₆₀₀ adjusted to 1.0) was examined at different times during growth (line) and corresponds to an average of three independent experiments. A representative growth profile is presented; OD₆₀₀ values in the three independent experiments differed by <40%. S, static (white bar and white triangle); S H100, static plus hemin (dark gray bar and dark gray triangle); A, aeration (light gray bar and light gray circle); A H100, aeration plus hemin (black bar and black circle). (B) Inactivation of ygf genes results in heme sensitivity. Stationary cultures of L. lactis strain MG1363 and ygfB (results not shown) and ygfC mutants were plated at 1/100 dilutions on M17G agar, and 20, 40, or 100 nmol of heme were pipetted directly onto plates; representative results with 40 nmol of heme are shown. A growth inhibition zone by heme (delimited by arrows) was substantially greater for the ygf mutant strains than for the parental strain and complemented mutants. The ring beyond the inhibition zone corresponds to stimulated growth due to respiration in the presence of heme.