Chicken beta B1-crystallin gene expression: presence of conserved functional polyomavirus enhancer-like and octamer binding-like promoter elements found in non-lens genes

Abstract

Expression of the chicken beta B1-crystallin gene was examined. Northern (RNA) blot and primer extension analyses showed that while abundant in the lens, the beta B1 mRNA is absent from the liver, brain, heart, skeletal muscle, and fibroblasts of the chicken embryo, suggesting lens specificity. Promoter fragments ranging from 434 to 126 bp of 5'-flanking sequence (plus 30 bp of exon 1) of the beta B1 gene fused to the bacterial chloramphenicol acetyltransferase gene functioned much more efficiently in transfected embryonic chicken lens epithelial cells than in transfected primary muscle fibroblasts or HeLa cells. Transient expression of recombinant plasmids in cultured lens cells, DNase I footprinting, in vitro transcription in a HeLa cell extract, and gel mobility shift assays were used to identify putative functional promoter elements of the beta B1-crystallin gene. Sequence analysis revealed a number of potential regulatory elements between positions -126 and -53 of the beta B1 promoter, including two Sp1 sites, two octamer binding sequence-like sites (OL-1 and OL-2), and two polyomavirus enhancer-like sites (PL-1 and PL-2). Deletion and site-specific mutation experiments established the functional importance of PL-1 (-116 to -102), PL-2 (-90 to -76), and OL-2 (-75 to -68). DNase I footprinting using a lens or a HeLa cell nuclear extract and gel mobility shifts using a lens nuclear extract indicated the presence of putative lens transcription factors binding to these DNA sequences. Competition experiments provided evidence that PL-1 and PL-2 recognize the same or very similar factors, while OL-2 recognizes a different factor. Our data suggest that the same or closely related transcription factors found in many tissues are used for expression of the chicken beta B1-crystallin gene in the lens.