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Basonuclin is a zinc finger protein specific to basal keratinocytes and germ cells. In keratinocytes, basonuclin behaves as a stem cell marker and is thought to be a transcription factor that maintains proliferative capacity and prevents terminal differentiation. The human gene is located on chromosome 15. We have discovered in the chicken the existence of basonuclin 2, a basonuclin homolog. We also report the entire sequence of mouse and human basonuclin 2; the corresponding genes are located on mouse chromosome 4 and human chromosome 9. Although the amino acid sequence of basonuclin 2 differs extensively from that of basonuclin 1, the two proteins share essential features. Both contain three paired zinc fingers, a nuclear localization signal, and a serine stripe. The basonuclin 2 mRNA has a wider tissue distribution than the basonuclin 1 mRNA: it is particularly abundant in testis, kidney, uterus, and intestine. The extreme conservation of the basonuclin 2 amino acid sequence across vertebrates suggests that basonuclin 2 serves an important function, presumably as a regulatory protein of DNA transcription.

Basonuclin (bn) 1 possesses three separated pairs of zinc fingers and a nuclear localization signal. It is largely confined to the basal cells of stratified squamous epithelia and to reproductive germ cells. bn1 can shuttle between the nucleus and the cytoplasm, and its location is correlated with the proliferative potential of the cell. The recently discovered bn2 also possesses three separated pairs of zinc fingers and a nuclear localization signal. Conservation of the zinc fingers and the nuclear localization signal by bn1 and bn2 indicates a common origin. However, in contrast to bn1, bn2 is found in virtually every cell type and is confined to the nucleus. Bn2 but not bn1 colocalizes with SC35 in nuclear speckles and, therefore, is likely to have a function in nuclear processing of mRNA.RNA processing ͉ speckles ͉ zinc fingers B asonuclin (bn1) is a zinc finger protein with highly restricted tissue distribution: It is found mainly in basal keratinocytes of stratified squamous epithelium and in reproductive germ cells (1-5). bn1 possesses three separated pairs of zinc fingers, a nuclear localization signal (NLS) and a serine-rich region that has been called the serine stripe (1). Although the evidence is not conclusive, it seems that the function of bn1 is related to the potential for cell proliferation (6). The only known function of bn1 is that of a transcription factor in the synthesis of ribosomal RNA (7, 8), but it is possible that bn1 possesses a nucleoplasmic function in the regulation of expression of genes transcribed by RNA polymerase II (9).A gene encoding a second basonuclin (bn 2) recently has been discovered (10, 11). Although the deduced amino acid sequences of bn1 and bn2 are only slightly Ͼ40% identical, bn2 possesses all of the characteristic features of bn1 described above. The bn2 mRNA is abundant in cell types that possess bn1 but is also found in tissues that lack bn1, such as kidney, intestine, and uterus. The genes for bn1 and bn2 differ greatly in size and are located on different chromosomes, but it is clear that they have a common evolutionary origin. The evolutionary conservation of bn2 is much greater than that of bn1. It has been postulated that the gene for bn2 is the older of the two. After its duplication, the gene for bn1 was free to evolve in other directions, whereas the gene for bn2 remained virtually invariant (9, 11).Previous work on immunocytological detection of bn1 was carried out with polyclonal antisera raised against most or all of the bn1 protein (6, 12). In view of the similarities between bn1 and bn2, it was possible that these antisera did not distinguish between the two basonuclins. To detect each basonuclin independently, we generated antibodies to specific determinants not shared by bn1 and bn2. Using these antibodies, we show that, although the two basonuclins are of common origin, they possess widely different functions, because bn2 is likely to have a function in pre-mRNA processing. ResultsEvolutionary Conservation of bn2. We had reported that bn2 was e...