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The aryl hydrocarbon receptor nuclear translocator (ARNT) is a basic-helix-loop-helix/Per- ARNT-Sim (bHLH/PAS) transcription factor that is involved in multiple signaling pathways. This study focuses on the tissue distribution and subcellular localization of ARNT during embryological development of the mouse and chicken. Two different species were chosen to determine the consistency of the ARNT staining pattern. Immunohistochemical techniques were used to stain sections of embryos over three developmental time points for each species. Mouse tissues evaluated from embryonic day 10.5, 12.5, and 15, exhibited predominant nuclear staining with little change in expression patterns over time. Chicken tissues evaluated from embryonic day 2, 4, and 10 also showed predominant nuclear staining within all cells and little change in expression over developmental time, as well as, low levels of cytoplasmic ARNT staining in some cells. Importantly, in all tissues, the level of ARNT staining within the nuclear compartment was greater than staining observed in the cytoplasm. Thus, the overall conclusions from these studies are that i) the predominant subcellular localization of ARNT protein is nuclear, and ii) that mouse and chicken appear to maintain ARNT protein expression in many cell types over developmental time. These data support vertebrate ARNT as a nuclear transcription factor and a model in which dimerization partners require nuclear localization for interaction., (Copyright 2000 Wiley-Liss, Inc.)

We have identified and characterized the gene for a novel zinc finger transcription factor which we have termed lung Krüppel-like factor (LKLF). LKLF was isolated through the use of the zinc finger domain of erythroid Krüppel-like factor (ELKF) as a hybridization probe and is closely related to this erythroid cell-specific gene. LKLF is expressed in a limited number of tissues, with the predominant expression seen in the lungs and spleen. The gene is developmentally controlled, with expression noted in the 7-day embryo followed by a down-regulation at 11 days and subsequent reactivation. A high degree of similarity is noted in the zinc finger regions of LKLF and EKLF. Beyond this domain, the sequences diverge significantly, although the putative transactivation domains for both LKLF and EKLF are proline-rich regions. In the DNA-binding domain, the three zinc finger motifs are so closely conserved that the predicted DNA contact sites are identical, suggesting that both proteins may bind to the same core sequence. This was further suggested by transactivation assays in which mouse fibroblasts were transiently transfected with a human beta-globin reporter gene in the absence and presence of an LKLF cDNA construct. Expression of the LKLF gene activates this human beta-globin promoter containing the CACCC sequence previously shown to be a binding site for EKLF. Mutation of this potential binding site results in a significant reduction in the reporter gene expression. LKLF and EKLF can thus be grouped as members of a unique family of transcription factors which have discrete patterns of expression in different tissues and which appear to recognize the same DNA-binding site.