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Journal of Bacteriology, October 1999, p. 6033-6041, Vol. 181, No. 19
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

A Mutant HemA Protein with Positive Charge Close to the N Terminus Is Stabilized against Heme-Regulated Proteolysis in Salmonella typhimurium

Liying Wang, Sandra Wilson, and Thomas Elliott*

Department of Microbiology and Immunology, West Virginia University Health Sciences Center, Morgantown, West Virginia 26506

Received 21 May 1999/Accepted 28 July 1999

The HemA enzyme (glutamyl-tRNA reductase) catalyzes the first committed step in heme biosynthesis in the enteric bacteria. HemA is mainly regulated by conditional protein stability; it is stable and, consequently, more abundant in heme-limited cells but unstable and less abundant in normally growing cells. Both the Lon and ClpAP energy-dependent proteases contribute to HemA turnover in vivo. Here we report that the addition of two positively charged lysine residues to the third and fourth positions at the HemA N terminus resulted in complete stabilization of the protein. By contrast, the addition of an N-terminal myc epitope tag did not affect turnover. This result confirms the importance of the N-terminal sequence for proteolysis of HemA. This region of the protein also contains a proline flanked by hydrophobic residues, a motif that has been suggested to be important for Lon-mediated proteolysis of UmuD. However, mutation of this motif did not affect the turnover of HemA protein. Cells expressing the stabilized HemA[KK] mutant protein display substantial defects in heme regulation.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, P.O. Box 9177, WVU Health Sciences Center, Morgantown, WV 26506-9177. Phone: (304) 293-2676. Fax: (304) 293-7823. E-mail: telliott{at}wvu.edu.

Journal of Bacteriology, October 1999, p. 6033-6041, Vol. 181, No. 19
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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