THE NTPJ GENE IN THE ENTEROCOCCUS-HIRAE NTP OPERON ENCODES A COMPONENT OF KTRII POTASSIUM-TRANSPORT SYSTEM FUNCTIONALLY INDEPENDENT OF VACUOLAR NA-ATPASE()

The ntpJ gene, the tail end in the vacuolar type Na+-ATPase (ntp) oper on of Enterococcus hirae, encodes a putative 49-kDa hydrophobic protei n resembling K+ transporter protein in Saccharomyces cerevisiae (Takas e, K., Kakinuma, S., Yamato, I., Konishi, K., Igarashi, K., and Kakinu ma, Y. (1994) J. Biol. Chem. 269, 11037-11044). Northern blotting expe riment revealed that the ntpJ gene was transcribed as a cistron in the ntp operon. We constructed an Enterococcus strain in which the ntpJ g ene was disrupted by cassette mutagenesis with erythromycin resistance gene. The growth of this mutant was normal at low pH. However, the mu tant did not grow at high pH in K+-limited medium (less than 1 mM), wh ile the wild type strain grew well; the internal K+ concentration of t his mutant was as low as 7% of that of the wild type strain, suggestin g that the K+ accumulation at high pH was inactivated by disruption of the ntpJ gene. Potassium uptake activity via the KtrII system, which had been proposed as the proton potential-independent, Na+-ATPase-coup led system working at high pH (Kakinuma, Y., and Harold, F. M. (1985) J. Biol. Chem. 260, 2086-2091), was missing in this mutant strain. How ever this mutant retained as high activities of Na+-ATPase and Na+ pum ping as the wild type strain. From these results, we conclude that the NtpJ is a membraneous component of the KtrII K+ uptake system but not a functional subunit of vacuolar Na+-ATPase complex; the interplay be tween the KtrII system and the Na+-ATPase was discussed.