Fig. 1. Growth and viability of the pVEII-AS5' transformants. (A) The pVEII-AS5'-1 transformant () and the Ax-2 parent ([UNK]), which were grown in shake suspension HL-5 growth media in the presence of 1 mM folate, were washed and transferred to fresh HL5 media in the absence of folate at a cell density of 1x10⁵ cells/ml. After three days of growth in shake suspension, when the population was in late exponential phase at a density of 5x10⁶ cells/ml, it was diluted into fresh HL-5 media to a density of 1x10⁵ cells/ml and growth continued. Growth was monitored by hemocytometer counts ([UNK],). (B) The viability of the pVEII-AS5' transformant () and Ax-2 ([UNK]) was determined by plating on rich nutrient agar plates in association with Klebsiella oxytoca. Cell viability is expressed relative to the viability at the start of the experiment and is the mean±s.e. for two experiments.