Fig. 1.

Complementation of DHFR-negative phenotype in a bacterial system. E. coli D3-157 streptomycin-resistant cells were transformed with either DHFR or DHFRL1 and grown in media with and without supplements/antibiotics. Strep, streptomycin 100 μg/mL; Amp, ampicillin 100 μg/mL; IPTG = 0.2 mM. Growth was measured at various time points until stationary phase was reached. (A) The original strain grew only in media containing thymidine and without ampicillin. (B) Cells transformed with empty vector only grew in the presence of thymidine with or without ampicillin and/or IPTG. (C) Cells transformed with DHFR grew as expected in media both with and without supplements, i.e., could grow in the absence of thymidine once DHFR expression was induced. (D) Cells transformed with DHFRL1 also complemented the phenotype similarly to recombinant DHFR.