Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights Reserved.): Colonies of Escherichia coli purD mutants with blue papillae representing independent IS903 transposition events. Cells contain an IS903 transposon carrying a cryptic lacZ gene, which can be expressed only following transposition into a host gene in the correct orientation and reading frame. Cells within a colony that have undergone transposition express β-galactosidase and are visualized by the addition of 5-bromo-4-chloro-3-indolyl-frame-β-D-galactopyranoside and lactose to the medium. Mutants deficient in purine biosynthesis have a distinct ring of papillae, in contrast to a random dispersal of papillae in wild-type colonies. This observation, together with other data, is evidence of a GTP requirement for transposition. (See related article on page 4598.)
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