Requirement for βB1-crystallin promoter of Xenopus laevis in embryonic lens development and lens regeneration

Abstract

Regulation of the lens-specific βB1-crystallin promoter in Xenopus laevis was investigated using transgenic larvae and tadpoles. Comparison of the promoter sequence with that of chicken βB1-crystallin gene indicates significant sequence similarity over a span of several hundred base pairs starting from the transcriptional start site. Remarkably, PL-1 and PL-2 sequences identified in the chicken promoter as essential binding sites of MAF, Pax6 and Prox1 transcription factors were conserved. Mutations of X (Xenopus) PL-1 and XPL-2 sequences eliminated the promoter activity, indicating a conserved mechanism regulating βB1-crystallin promoter among vertebrate species. A stepwise deletion of the promoter sequence starting from 2800 bp indicated that the proximal 260 bp directly upstream of the transcription initiation site is sufficient for eliciting lens-specific expression, but the 150 bp promoter sequence is inactive despite it containing the XPL-1 and XPL-2 sequences, suggesting the presence of an additional and essential regulatory sequence located between -150 and -260 bp. Activity of the βB1-crystallin promoter during lens regeneration from cornea was examined using transgenic tadpoles and found to have the same dependence on promoter regions as in embryonic lens development, indicating that gene regulation is largely shared by the two lens-generating processes.