Background : The inhibition of DNA replication fork progression by DNA lesions can lead to cell death or genome instability. However, little is known about how such DNA lesions affect the concurrent synthesis of leading-and lagging-strand DNA catalysed by the protein machinery used in chromosomal replication. Using a system of semi-bidirectional DNA replication of an oriC plasmid that employs purified replicative enzymes and a replication-terminating protein of Escherichia coli , we examined the dynamics of the replication fork when it encounters a single abasic DNA lesion on the template DNA.

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Nutritional factors were evaluated for effects on growth of mouse fibroblast cells in suspension in a chemically defined medium. Quantitative requirements for each of the essential amino acids, choline, inorganic phosphate, iron, and zinc were established. An improved Chemically defined medium was formulated on the basis o f the findings yielding populations of L cells in excess of 5 x lo6 per ml without nutrient replenishment. When spent medium was replaced periodically, yields approaching 30 x lo6 cells per ml were attained. The efficiency of utilization of most amino acids in the new medium appears to be 2-to 3-fold better than results reported by others.