We studied the relative RNA expression of clock genes throughout one 24-hour period in biopsies obtained from the oral mucosa and skin from eight healthy diurnally active male study participants. We found that the human clock genes hClock, hTim, hPer1, hCry1, and hBmal1 are expressed in oral mucosa and skin, with a circadian profile consistent with that found in the suprachiasmatic nuclei and the peripheral tissues of rodents. hPer1, hCry1, and hBmal1 have a rhythmic expression, peaking early in the morning, in late afternoon, and at night, respectively, whereas hClock and hTim are not rhythmic. This is the first human study to show a circadian profile of expression for all five clock genes as documented in rodents, suggesting their functional importance in man. In concurrent oral mucosa biopsies, thymidylate synthase enzyme activity, a marker for DNA synthesis, had a circadian variation with peak activity in early afternoon, coinciding with the timing of S phase in our previous study on cell-cycle timing in human oral mucosa. The major peak in hPer1 expression occurs at the same time of day as the peak in G(1) phase in oral mucosa, suggesting a possible link between the circadian clock and the mammalian cell cycle.

The retroviral integration siteThe hematopoietic system is composed of a hierarchy of cells, ranging from the pluripotent stem cell, committed myeloid and lymphoid progenitor cells, to terminally differentiated blood cells (Metcalf 1988;Sachs 1987). The molecular mechanisms that control the developmental and proliferative decisions of the pluripotent hematopoietic stem cell are largely unknown. The extreme cellular heterogeneity of hematopoietic populations, combined with the low frequency of stem cells, has hampered experimental approaches to this problem. The generation of mutations that affect stem cell function and the molecular analysis of existing developmental mutations that control hematopoiesis (Pawson and Bernstein 1990) provide two powerful genetic strategies for identifying and characterizing the genes that control normal blood cell formation. In addition, because cellular genes involved in leukemic transformation may also have a role in regulating normal stem cell function, the identification of such genes may provide insights into both normal and leukemic hematopoiesis. Transformation by certain retroviruses can involve the integration of proviral DNA near or within specific cellular genes. Such integration events most frequently lead to the elevated expression of these genes (for review, see in Peters 1990), although the tumor suppressor gene p53 can also be inactivated by retroviral integration of the Friend leukemia virus (Ben-David et al. 1988, 1990b. The analysis of common retroviral integration events can, therefore, be used to identify and clone novel transforming genes involved in tumorigenesis. Using this approach, a large number of genes important in the leukemic transformation of hematopoietic cells, including have been isolated (Peters 1990).The Fli-1 and Spi-1 genes have been shown by us and others (Moreau-Gachelin et al. 1988; Ben-David et al. 1990) to be involved in erythroleukemia induction by various strains of Friend leukemia virus. Fli-1 (Friend leukemia integration-site 1) is rearranged in 75% of independently isolated erythroleukemic clones from mice inoculated at birth with the replication-competent Cold Spring Harbor Laboratory Press on May 9, 2018 -Published by genesdev.cshlp.org Downloaded from

Telomeric DNA of mammalian chromosomes consists of several kilobase-pairs of tandemly repeated sequences with a terminal 3' overhang in single-stranded form. Maintaining the integrity of these repeats is essential for cell survival; telomere attrition is associated with chromosome instability and cell senescence, whereas stabilization of telomere length correlates with the immortalization of somatic cells. Telomere elongation is carried out by telomerase, an RNA-dependent DNA polymerase which adds single-stranded TAGGGT repeats to the 3' ends of chromosomes. While proteins that associate with single-stranded telomeric repeats can influence tract lengths in yeast, equivalent factors have not yet been identified in vertebrates. Here, it is shown that the heterogeneous nuclear ribonucleoprotein A1 participates in telomere biogenesis. A mouse cell line deficient in A1 expression harbours telomeres that are shorter than those of a related cell line expressing normal levels of A1. Restoring A1 expression in A1-deficient cells increases telomere length. Telomere elongation is also observed upon introduction of exogenous UP1, the amino-terminal fragment of A1. While both A1 and UP1 bind to vertebrate single-stranded telomeric repeats directly and with specificity in vitro, only UP1 can recover telomerase activity from a cell lysate. These findings establish A1/UP1 as the first single-stranded DNA binding protein involved in mammalian telomere biogenesis and suggest possible mechanisms by which UP1 may modulate telomere length.