Missing self recognition of MHC class I molecules is mediated in murine species primarily through the stochastic expression of CD94/NKG2 and Ly49 receptors on NK cells. Previous studies have suggested that the stochastic expression of Ly49 receptors is achieved through the use of an alternate upstream promoter, designated Pro1, that is active only in immature NK cells and operates via the mutually exclusive binding of transcription initiation complexes to closely opposed forward and reverse TATA boxes, with forward transcription being transiently required to activate the downstream promoters, Pro2/Pro3, that are subsequently responsible for transcription in mature NK cells. In this study, we report that Pro1 transcripts are not restricted to immature NK cells but are also found in mature NK cells and T cells, and that Pro1 fragments display strong promoter activity in mature NK cell and T cell lines as well as in immature NK cells. However, the strength of promoter activity in vitro does not correlate well with Ly49 expression in vivo and forward promoter activity is generally weak or undetectable, suggesting that components outside of Pro1 are required for efficient forward transcription. Indeed, conserved sequences immediately upstream and downstream of the core Pro1 region were found to inhibit or enhance promoter activity. Most surprisingly, promoter activity does not require either the forward or reverse TATA boxes, but is instead dependent on residues in the largely invariant central region of Pro1. Importantly, Pro1 displays strong enhancer activity, suggesting that this may be its principal function in vivo.
Copyright © 2015 by The American Association of Immunologists, Inc.