Escherichia coli SecA helicase activity is not required in vivo for efficient protein translocation or autogenous regulation

SecA is an essential ATP-driven motor protein that binds to preproteins and the translocon to promote protein translocation across the eubacterial pla sma membrane. Escherichia coli SecA contains seven conserved motifs charact eristic of superfamily II of DNA and RNA helicases, and it has been shown p reviously to possess RNA helicase activity. SecA has also been shown to be an autogenous repressor that binds to its translation initiation region on secM-secA mRNA, thereby blocking, and dissociating 30 S ribosomal subunits. Here we show that SecA is an ATP-dependent helicase that unwinds a mimic o f the repressor helix of secM-secA mRNA. Mutational analysis of the seven c onserved helicase motifs in SecA allowed us to identify mutants that uncoup le SecA-dependent protein translocation activity from its helicase activity . Helicase-defective secA mutants displayed normal protein translocation ac tivity and autogenous repression of secA in vivo. Our studies indicate that SecA helicase activity is nonessential and does not appear to be necessary for efficient protein secretion and secA autoregulation.