FIG 3.

Effect of osmolality on the ability of R67 and Quad4 DHFR to rescue E. coli DH5α from trimethoprim. Panel A shows the effect of sorbitol on cell growth in M9 minimal medium lacking TMP. Cells can grow up to 1.90 osmol, although growth is slow and sparse. Panel B shows the effect of sorbitol on growth in media containing 20 μg/ml TMP, 100 μg/ml ampicillin, and 100 ng/ml tetracycline. Cells carrying the R67 DHFR-pKTS clone can grow only until osmolalities between 0.81 and 1.28 are reached, while cells carrying the Quad4-pKTS clone can grow until osmolalities between 1.60 and 1.93 are reached. Table S1 lists the growth patterns as a function of osmolality. The drawings at the right show how various cultures were streaked on the plates. Two independent colonies were streaked per transformant.