Worm Breeder's Gazette 14(1): 71 (October 1, 1995)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Carnegie Institution, Baltimore MD 21210.
Thanks to Kim Ferguson and Joel Rothman for gifts of SL1 plasmid and zen-1 strains. We have previously described the embryonic distribution of RNAs containing the splice leader SL1 (Seydoux and Fire, 1994, Development 120, p. 2823). Using an oligo complementary to the 22 nucleotides of SL1 that are transpliced, we had detected SL1-containing RNAs in the cytoplasm of all cells, and in P granules, from the one-cell stage to the pretzel stage. We have now examined the distribution of the full-length SL1 precursor RNA. To do this, we used a probe complementary to the SL1 precursor RNA, under conditions where hybridization to the 22bp trans-spliced leader would not occur. We detected SL1 precursor RNA in the germline syncytium, in oocyte nuclei, and in interphase and prophase nuclei of early embryos (1-28/50 cell stage). No SL1 precursor RNA was detected past the 50-cell stage, except for occasional staining in 1 or 2 nuclei, which we tentatively identified as the nuclei of P4, and Z2/Z3. To test whether the SL1 precursor RNA detected in embryos was maternal or zygotic, we repeated our in situ hybridization using embryos derived from unc-76(e911) zen-1(e2482); wEx127 (pRF4, SL1/5SrRNA) hermaphrodites. These hermaphrodites segregate 65% viable embryos (which carry the SL1 gene on an extrachromosomal array) and 35% dead embryos (which presumably have lost the array and have no copy of SL1). If the embryonic distribution of SL1 was due to zygotic transcription, we would have observed 35% of embryos with no staining. Instead, we found that 95% of 1-28-cell stage embryos were positive for the SL1 precursor, indicating that this early RNA is independent of zygotic transcription. This result suggests that SL1 precursor RNA synthesized in the germline is inherited by the embryo and repeatedly localized to the nucleus following each mitosis. What about zygotic transcription of SL1? Kim Ferguson and Joel Rothman have demonstrated a zygotic requirement for SL1 as early as the 28-cell stage (wm95, p4). It appears, therefore, that SL1 is transcribed in the early embryo, in addition to being maternally inherited. We have not detected SL1 precursor RNA in somatic nuclei past the 50-cell stage: one possibility is that, in these later stages, SL1 precursor RNA no longer accumulates to detectable levels as increased numbers of zygotic transcripts are available for trans-splicing.