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Growth in the presence and absence of thymidine of a thyA mutant of Escherichia coli transformed with the plasmid vector pUC18 without an insert (open triangles) or with an insert of thyA of E. coli (solid squares), thyA of Lactobacillus casei (solid triangles), thyX of Borrelia burgdorferi (open circles), or thyX of B. hermsii (solid diamonds). Growth was determined by measurement of the optical density at 600 nm by a spectrophotometer.
