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Journal of Bacteriology, June 2004, p. 3472-3479, Vol. 186, No. 11
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.11.3472-3479.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Microbiology and Immunology, University of Michigan, Ann Arbor, Michigan 48109-0620
Received 15 December 2003/ Accepted 11 February 2004
The Shiga toxin (Stx)-encoding bacteriophage 933W contains an open reading frame, stk, with amino acid sequence similarity to the catalytic domain of eukaryotic serine/threonine (Ser/Thr) protein kinases (PKs). Eukaryotic PKs are related by a common catalytic domain, consisting of invariant and nearly invariant residues necessary for ATP binding and phosphotransfer. We demonstrate that rather than a Ser/Thr kinase, stk encodes a eukaryotic-like tyrosine (Tyr) kinase. An affinity-purified recombinant Stk (rStk) autophosphorylates and catalyzes the phosphorylation of an artificial substrate on Tyr residues and not on Ser or Thr residues. A change of an invariant lysine within the putative catalytic domain abolishes this kinase activity, indicating that Stk uses a phosphotransfer mechanism similar to the mechanism used by eukaryotic PKs. We provide evidence suggesting that stk is cotranscribed with cI from the phage promoter responsible for maintaining CI expression during lysogeny. The stk gene was identified in prophages obtained from independently isolated Stx-producing Escherichia coli clinical isolates, suggesting that selective pressure has maintained the stk gene in these pathogenic bacteria.
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