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Journal of Bacteriology, May 2004, p. 2774-2780, Vol. 186, No. 9
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.9.2774-2780.2004

The {theta} Subunit of Escherichia coli DNA Polymerase III: a Role in Stabilizing the {varepsilon} Proofreading Subunit

Sharon A. Taft-Benz{dagger} and Roel M. Schaaper*

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709

Received 18 December 2003/ Accepted 16 January 2004

The function of the {theta} subunit of Escherichia coli DNA polymerase III holoenzyme is not well established. {theta} is a tightly bound component of the DNA polymerase III core, which contains the {alpha} subunit (polymerase), the {varepsilon} subunit (3'->5' exonuclease), and the {theta} subunit, in the linear order {alpha}-{varepsilon}-{theta}. Previous studies have shown that the {theta} subunit is not essential, as strains carrying a deletion of the holE gene (which encodes {theta}) proved fully viable. No significant phenotypic effects of the holE deletion could be detected, as the strain displayed normal cell health, morphology, and mutation rates. On the other hand, in vitro experiments have indicated the efficiency of the 3'-exonuclease activity of {varepsilon} to be modestly enhanced by the presence of {theta}. Here, we report a series of genetic experiments that suggest that {theta} has a stabilizing role for the {varepsilon} proofreading subunit. The observations include (i) defined {Delta}holE mutator effects in mismatch-repair-defective mutL backgrounds, (ii) strong {Delta}holE mutator effects in certain proofreading-impaired dnaQ strains, and (iii) yeast two- and three-hybrid experiments demonstrating enhancement of {alpha}-{varepsilon} interactions by the presence of {theta}. {theta} appears conserved among gram-negative organisms which have an exonuclease subunit that exists as a separate protein (i.e., not part of the polymerase polypeptide), and the presence of {theta} might be uniquely beneficial in those instances where the proofreading 3'-exonuclease is not part of the polymerase polypeptide.

* Corresponding author. Mailing address: NIEHS, Laboratory of Molecular Genetics, MD E3-01, P.O. Box 12233, 111 T. W. Alexander Dr., Research Triangle Park, NC 27709. Phone: (919) 541-4250. Fax: (919) 541-7613. E-mail: schaaper{at}niehs.nih.gov.

{dagger} Present address: Department of Microbiology and Immunology, University of North Carolina, Chapel Hill, NC 27599.

Journal of Bacteriology, May 2004, p. 2774-2780, Vol. 186, No. 9
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.9.2774-2780.2004




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