Many of the developed
expression systems have inducible promoters whose
expression depends on environmental conditions. These include,
for example, the P170 promoter, active at low pH and
subject to self-induction by accumulated lactic acid when the
culture enters the stationary phase of growth (Madsen et al.
1999), or the zit operon promoter that is regulated by ZitR
protein and is activated by low concentration of zinc ions
(the so-called zinc hunger) (Llull and Poquet 2004).