TABLE 4.

An mtrR mutation is required for expression of penB-mediated penicillin and nafcillin resistance^a

Strain	mtrR	pen	MIC (μg/ml)
			TET	PEN	NAF	TX-100
FA19	+	−	0.25	0.015	0.25	250
FA19AB	+	penA4, penB	0.25	0.06	1.0	250
WV38	mtrR171	penA4, penB	1.0	0.5	8.0	>16,000

^aAll abbreviations are as described in Table 2, footnote a. MICs are representative values from at least three determinations. +, wild type; −, no mutation in pen genes (see Table 1, footnote a).