Novel functions of nanos in downregulating mitosis and transcription during the development of the Drosophila germline

It has previously been shown that germ cells in embryos derived from nos mutant mothers do not migrate to the primitive gonad and prematurely express several germline-specific markers. In the studies reported here, we have traced these defects back to the syncytial blastoderm stage. We show that pole cells in nos embryos fail to establish/maintain transcriptional quiescence; the sex determination gene Sex-lethal (Sxl) and the segmentation genes fushi tarazu and even-skipped are ectopically activated in nos- germ cells. We show that nos- germ cells are unable to attenuate the cell cycle and instead continue dividing. Unexpectedly, removal of the Sxl gene in the zygote mitigates both the migration and mitotic defects of nos- germ cells. Supporting the conclusion that Sxl is an important target for nos repression, ectopic, premature expression of Sxl protein in germ cells disrupts migration and stimulates mitotic activity.

An N-terminal truncation uncouples the sex-transforming and dosage compensation functions of sex-lethal

In Drosophila melanogaster, Sex-lethal (Sxl) controls autoregulation and sexual differentiation by alternative splicing but regulates dosage compensation by translational repression. To elucidate how Sxl functions in splicing and translational regulation, we have ectopically expressed a full-length Sxl protein (Sx.FL) and a protein lacking the N-terminal 40 amino acids (Sx-N). The Sx.FL protein recapitulates the activity of Sxl gain-of-function mutations, as it is both sex transforming and lethal in males. In contrast, the Sx-N protein unlinks the sex-transforming and male-lethal effects of Sxl. The Sx-N proteins are compromised in splicing functions required for sexual differentiation, displaying only partial autoregulatory activity and almost no sex-transforming activity. On the other hand, the Sx-N protein does retain substantial dosage compensation function and kills males almost as effectively as the Sx.FL protein. In the course of our analysis of the Sx.FL and Sx-N transgenes, we have also uncovered a novel, negative autoregulatory activity, in which Sxl proteins bind to the 3' untranslated region of Sxl mRNAs and decrease Sxl protein expression. This negative autoregulatory activity may be a homeostasis mechanism.

Stringent sequence requirements for the formation of human telomeres

In human cells, transfection of telomeric T2AG3 repeats induces the formation of functional telomeres at previously interstitial sites. We report that telomere formation has stringent sequence requirements. While (T2AG3)n telomere seeds formed telomeres in approximately 70% of the transfected cells, five T2AG3-related heterologous telomeric DNAs seeded new telomeres in < 5% of the transfectants. Telomere formation did not correlate with the ability of human telomerase to elongate telomeric sequences in vitro. Homologous recombination is probably also not involved because a (T2AG3)n telomere seed with nontelomeric DNA at 160-bp intervals formed new telomeres frequently. Instead, the sequence dependence of telomere formation matched the in vitro binding requirements for the mammalian T2AG3 repeat binding factor (TRF). Human TRF failed to bind ineffective heterologous telomere seeds and had a 4-fold lower affinity for (T2AG5)2T2AG3 repeats, which seeded telomeres with reduced frequency. The results suggest that telomere seeds interact with TRF and predict that mammalian artificial chromosomes will require wild-type telomeric repeats at, or near, their termini.