Mutations in any one of three genes, kdpA, -B, or -C, in Escherichia coli abolish the activity of Kdp, a multisubunit K+-ATPase that belongs to the P-type ATPase family of cation transporters. We found in this study that expression in vivo of a 135-amino-acid-long N-terminal fragment (KdpA′), less than one-quarter the length of native KdpA, was able to mediate an improvement in K+-limited growth rates in two different contexts, even in the absence of both KdpC and the ATPase subunit KdpB. The first context was when KdpA′ was overexpressed in cells from a heterologous inducible promoter, and the second was when KdpA′ was provided with a C-terminally altered extension (following a spontaneous genetic rearrangement). Our results suggest that KdpA′ provides an incipient pathway for K+ translocation which can serve to transport K+ into the cells in response to the cytoplasmic membrane potential
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