Characterization of the Core Promoter of the Na+/K+‐ATPase α1 Subunit Gene

We have analyzed the core promoter element of the Na + /K + ‐ATPase α1 subunit gene by means of an in vitro transcription system composed of a HeLa nuclear extract. 5′‐deletion and 3′‐deletion analyses revealed that this gene is specifically transcribed by RNA polymerase II in a manner that is dependent on the upstream regulatory region of the gene (−102 to −61), and that the 3′ boundary of the minimal promoter element does not extend beyond +5. Analysis of linker‐substitution mutations and point mutations revealed that the TATA‐like sequence (−33 to −26) is required for upstream‐sequence‐dependent transcription whereas linker‐substitution mutations and point mutations near +1 did not abolish transcription. The gene was found to be transcribed by RNA polymerase III when phosphocellulose column fractions were assayed. Deletion analysis mapped the minimal RNA‐polymerase‐III–specific promoter element from −49 to +17. The phosphocellulose 0.3‐M‐KCl fraction is absolutely required for transcription by RNA polymerase III, while the 0.85‐M‐KCl fraction represses aberrant transcription from incorrect initiation sites. Analysis of linker‐substitution mutations indicated that the TATA‐like sequence is required for RNA‐polymerase‐III–specific transcription. Although point mutations in the 5′ half of the TATA‐like sequence did not affect transcription, those in the 3′ half shifted the transcription initiation site 3 bp upstream. The results suggest that the Na + /K + ‐ATPase α1 subunit gene promoter contains a TATA‐like sequence which can direct transcription by RNA polymerase III in vitro. The mechanism of alternative regulation of RNA polymerase II and RNA polymerase III is discussed.