Published online 9 December 2005
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Coupling DNA-binding and ATP hydrolysis in Escherichia coli RecQ: role of a highly conserved aromatic-rich sequence
Department of Biomolecular Chemistry, 550 Medical Science Center 1300 University Avenue University of Wisconsin Medical School Madison WI 53706-1532, USA
*To whom correspondence should be addressed. Tel: +608 263 1815; Fax: +608 262 5253; Email: jlkeck{at}wisc.edu
Received August 15, 2005. Revised November 4, 2005. Accepted November 15, 2005.
RecQ enzymes are broadly conserved Superfamily-2 (SF-2) DNA helicases that play critical roles in DNA metabolism. RecQ proteins use the energy of ATP hydrolysis to drive DNA unwinding; however, the mechanisms by which RecQ links ATPase activity to DNA-binding/unwinding are unknown. In many Superfamily-1 (SF-1) DNA helicases, helicase sequence motif III links these activities by binding both single-stranded (ss) DNA and ATP. However, the ssDNA-binding aromatic-rich element in motif III present in these enzymes is missing from SF-2 helicases, raising the question of how these enzymes link ATP hydrolysis to DNA-binding/unwinding. We show that Escherichia coli RecQ contains a conserved aromatic-rich loop in its helicase domain between motifs II and III. Although placement of the RecQ aromatic-rich loop is topologically distinct relative to the SF-1 enzymes, both loops map to similar tertiary structural positions. We examined the functions of the E.coli RecQ aromatic-rich loop using RecQ variants with single amino acid substitutions within the segment. Our results indicate that the aromatic-rich loop in RecQ is critical for coupling ATPase and DNA-binding/unwinding activities. Our studies also suggest that RecQ's aromatic-rich loop might couple ATP hydrolysis to DNA-binding in a mechanistically distinct manner from SF-1 helicases.
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