FIG. 3.

Expression levels of estA in different L. lactis strains and constructs. B1014 and MG1363 are wild-type tributyrin esterase producer strains, NZ9000 and NZ9800 are nisin-sensitive strains for induction of gene expression, and NZ9340 is an esterase-deficient mutant. Plasmids pNZ9308 (B1014 estA) and pNZ9330 (MG1363 estA) are derivatives of pNZ8020 (transcriptional fusion vector), while pNZ9310 (B1014 estA) and pNZ9331 (MG1363 estA) are derivatives of pNZ8030 (translational fusion vector). The esterase activity was determined in cell extracts after induction with 3 ng of nisin A ml⁻¹ and in the absence of nisin A, and it is expressed as micromoles of p-nitrophenol liberated from p-NP-butanoate per minute per milligram of protein.