Enhancer-independent promoter activity of the mouse αB-crystallin/small heat shock protein gene in the lens and cornea of transgenic mice

The alpha B-crystallin/small heat shock protein gene is expressed very highly in the mouse eye lens and to a lesser extent in many other nonocular tissues, including the heart, skeletal muscle and brain. Previously we showed in transgenic mice that lens-specific alpha B-crystallin promoter activity is directed by a proximal promoter fragment (-164/+44) and that non-lens promoter activity depends on an upstream enhancer (-427/-259) composed of at least 5 cis-control elements. Here we have used truncated alpha B-crystallin promoter-CAT transgenes to test by biphasic CAT assays and/or histochemistry for specific expression in the cornea and lens. Deletion either of 87 bp (-427/-340) from the 5' end of the alpha B-crystallin enhancer or of the whole enhancer (-427/-258) abolished alpha B-crystallin promoter activity in all tissues except the lens and corneal epithelium when examined by the biphasic CAT assay in 4-5-week-old transgenic mice. These truncations also lowered promoter strength in the lens. The -426/+444-CAT, - 339/+44-CAT and - 164/+44-CAT (previously thought to be lens-specific in transgenic mice) transgenes were all expressed in the 4-6-week-old corneal epithelium when examined histochemically. Immunohistochemical staining confirmed the presence of endogenous alpha B-crystallin in the mature corneal epithelial cells. CAT gene expression driven by the alpha B-crystallin promoter with or without the enhancer was evident in the embryonic and 4-6-week-old lens. By contrast, activity of the alpha B-crystallin promoter/enhancer-CAT transgene was not detectable in the corneal epithelium before birth. Taken together, these results indicate that the intact enhancer of the alpha B-crystallin/small heat shock protein gene is required for promoter activity in all tissues tested except the lens and cornea. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.