Dual roles for Pax-6: a transcriptional repressor of lens fiber cell-specific beta-crystallin genes.
It has been demonstrated previously that Pax-6, a paired domain (PD)/homeodomain (HD) transcription factor critical for eye development, contributes to the activation of the alphaB-, alphaA-, delta1-, and zeta-crystallin genes in the lens. Here we have examined the possibility that the inverse relationship between the expression of Pax-6 and beta-crystallin genes within the developing chicken lens reflects a negative regulatory role of Pax-6. Cotransfection of a plasmid containing the betaB1-crystallin promoter fused to the chloramphenicol acetyltransferase reporter gene and a plasmid containing the full-length mouse Pax-6 coding sequences into primary embryonic chicken lens epithelial cells or fibroblasts repressed the activity of this promoter by as much as 90%. Pax-6 constructs lacking the C-terminal activation domain repressed betaB1-crystallin promoter activity as effectively as the full-length protein, but the PD alone or Pax-6 (5a), a splice variant with an altered PD affecting its DNA binding specificity, did not. DNase footprinting analysis revealed that truncated Pax-6 (PD+HD) binds to three regions (-183 to -152, -120 to -48, and -30 to +1) of the betaB1-crystallin promoter. Earlier experiments showed that the betaB1-crystallin promoter sequence from -120 to -48 contains a cis element (PL2 at -90 to -76) that stimulates the activity of a heterologous promoter in lens cells but not in fibroblasts. In the present study, we show by electrophoretic mobility shift assay and cotransfection that Pax-6 binds to PL2 and represses its ability to activate promoter activity; moreover, mutation of PL2 eliminated binding by Pax-6. Taken together, our data indicate that Pax-6 (via its PD and HD) represses the betaB1-crystallin promoter by direct interaction with the PL2 element. We thus suggest that the relatively high concentration of Pax-6 contributes to the absence of betaB1-crystallin gene expression in lens epithelial cells and that diminishing amounts of Pax-6 in lens fiber cells during development allow activation of this gene.
Department of Biological Sciences, The University of Delaware, Newark, Delaware 19716, USA.
M K
MK
Duncan
J I
JI
Haynes
A
A
Cvekl
J
J
Piatigorsky
D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D002460:Cell Line; D002642:Chick Embryo; D002645:Chickens; D015500:Chloramphenicol O-Acetyltransferase / Q000096:biosynthesis; D003001:Cloning, Molecular; D003459:Crystallins / Q000096:biosynthesis* / Q000235:genetics; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000378:metabolism*; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism; D005136:Eye Proteins / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D017930:Genes, Reporter; D018398:Homeodomain Proteins; D007908:Lens, Crystalline / Q000166:cytology* / Q000254:growth & development / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D011401:Promoter Regions, Genetic; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism; D014162:Transfection
D003459:Crystallins; D004268:DNA-Binding Proteins; D005136:Eye Proteins; D018398:Homeodomain Proteins; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; C000605732:Pax6 protein, mouse; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; D015500:Chloramphenicol O-Acetyltransferase