The DRE motif is a key component in the expression regulation of the importin-β encoding Ketel gene in Drosophila

β-importin Tnpo-SR promotes germline stem cell maintenance and oocyte differentiation in female Drosophila

Germ cell development requires interplay between factors that balance cell fate and division. Early in their development, germ cells in many organisms divide mitotically with incomplete cytokinesis. Key regulatory events then lead to the specification of mature gametes, marked by the switch to a meiotic cell cycle program. Though the regulation of germ cell proliferation and meiosis are well understood, how these events are coordinated during development remains incompletely described. Originally characterized in their role as nucleo-cytoplasmic shuttling proteins, β-importins exhibit diverse functions during male and female gametogenesis. Here, we describe novel, distinct roles for the β-importin, Transportin-Serine/Arginine rich (Tnpo-SR), as a regulator of the mitosis to meiosis transition in the Drosophila ovary. We find that Tnpo-SR is necessary for germline stem cell (GSC) establishment and self-renewal, likely by controlling the response of GSCs to bone morphogenetic proteins. Depletion of Tnpo-SR results in germ cell counting defects and loss of oocyte identity. We show that in the absence of Tnpo-SR, proteins typically suppressed in germ cells when they exit mitosis fail to be down-regulated, and oocyte-specific factors fail to accumulate. Together, these findings provide new insight into the balance between germ cell division and differentiation and identify novel roles for β-importins in germ cell development.

Control of e2f1 and PCNA by Drosophila transcription factor DREF

DREF (DNA replication-related element-binding factor), a zinc finger type transcription factor required for proper cell cycle progression in both mitotic and endocycling cells, is a positive regulator of E2F1, an important transcription factor which regulates genes related to the S-phase of the cell cycle. DREF and E2F1 regulate similar sets of replication-related genes, including proliferating cell nuclear antigen (PCNA), and play roles in the G1 to S phase transition. However, the relationships between dref and e2f1 or PCNA during development are poorly understood. Here, we provided evidence for novel control of e2f1 and PCNA involving DREF in endocycling cells. Somatic clone analysis demonstrated that dref knockdown stabilized E2F1 expression at posttranscriptional levels in endocycling salivary gland cells. Similarly, PCNA expression was up-regulated in the endocycling salivary gland cells. Genetic interaction analysis indicated that the endoreplication defects are partly caused via possible enhancement of E2F1 activity. From these results and previous reports, we conclude that regulation of e2f1 and PCNA by DREF in vivo is complex and the regulation mechanism may differ with the tissue and/or positions in the tissue.

The involvement of Importin-β and peroxiredoxin-6005 in mitochondrial biogenesis

Importin-β is encoded by the Ketel gene in Drosophila. Upon running out of the maternal Importin-β dowry larvae without the Ketel gene slow down and before dying possess symptoms characteristic for mitochondrial cytopathies. Death of the larvae is almost certainly the consequence of ceasing import of proteins, including some of the transcription factors, into the nuclei. We report here that the ensuing altered gene expression pattern leads to cessation of mitochondrial biogenesis. A transcriptome comparison between larvae with and without Ketel gene revealed altered expression level for 30 genes that are all nuclear. The seven downregulated genes have C/EBP transcription factor binding site in their promoter. RNAi silencing the function of peroxiredoxin-6005, one of the 23 upregulated genes, leads to excessive mitochondrial biogenesis, free radical production and death of the larvae. It appears that peroxiredoxin-6005 is engaged in mitochondrial biogenesis possibly as a component of redox-signaling.

The RNA Pol II CTD phosphatase Fcp1 is essential for normal development in Drosophila melanogaster

The reversible phosphorylation-dephosphorylation of RNA polymerase II (Pol II) large subunit carboxyl terminal domain (CTD) during transcription cycles in eukaryotic cells generates signals for the steps of RNA synthesis and maturation. The major phosphatase specific for CTD dephosphorylation from yeast to mammals is the TFIIF-interacting CTD-phosphatase, Fcp1. We report here on the in vivo analysis of Fcp1 function in Drosophila using transgenic lines in which the phosphatase production is misregulated. Fcp1 function is essential throughout Drosophila development and ectopic up- or downregulation of fcp1 results in lethality. The fly Fcp1 binds to specific regions of the polytene chromosomes at many sites colocalized with Pol II. In accord with the strong evolutional conservation of Fcp1: (1) the Xenopus fcp1 can substitute the fly fcp1 function, (2) similarly to its S. pombe homologue, Drosophila melanogaster (Dm)Fcp1 interacts with the RPB4 subunit of Pol II, and (3) transient expression of DmFcp1 has a negative effect on transcription in mammalian cells. The in vivo experimental system described here suggests that fly Fcp1 is associated with the transcription engaged Pol II and offers versatile possibilities for studying this evolutionary conserved essential enzyme.