Transcription factor CP2 is essential for lens-specific expression of the chicken αA-crystallin gene

Abstract

Background: Lens-specific transcriptional activation of the chicken αA- crystallin gene is controlled by the distal and proximal enhancers, αCE1 and αCE2, respectively. Analysis using specific monoclonal antibodies against purified αCE1-binding factor αCEF1 revealed that αCEF1 is composed of two distinct subunits. Results: We have demonstrated that one of the subunits of αCEF1 is encoded by chicken ubiquitous transcription factor CP2 (cCP2), which is homologous to mouse CP2, and human CP2/LBP-1/LSF-1. Electrophoretic mobility shift assays and cross-linking experiments showed that αCEF1 and bacterially expressed cCP2 form a tetramer. Overexpression of cCP2 activates transcription through αCE1, but a mutant cCP2 lacking the DNA-binding domain reduced the transcription to basal levels. Although cCP2 binds to the CP2 template from the mouse α-globin promoter, it fails to promote transcription through this template. Element substitution experiments between αCE1 and the CP2 template revealed that the lens-specific enhancer activity of αCE1 is due to the 6 bp sequence (-139/-134; lens-specific element (LSE)) adjacent to the 3' of the cCP2 binding site within αCE1. Conclusion: We have shown that the tetrameric transcription factor cCP2 is essential for lens-specific transcription of the chicken αA-crystallin gene, although it is ubiquitously expressed. We propose a model where cCP2 cooperates with a putative lens- specific factor which binds to LSE.