The three rows gene of Drosophila melanogaster encodes a novel protein that is required for chromosome disjunction during mitosis.

Abstract

Zygotic expression of the three rows (thr) gene of Drosophila melanogaster is required for normal cell proliferation during embryogenesis. Mitotic defects in thr mutant embryos begin during mitosis 15, and all subsequent divisions are disrupted. Chromosome disjunction and consequently cytokinesis fail during these defective mitoses, although the initial mitotic processes (chromosome condensation, spindle assembly, metaphase plate formation, and cyclin degradation) are not affected. Despite the failure of chromosome disjunction and cytokinesis, later mitotic events (chromosome decondensation) and subsequent cell cycle progression continue. The thr gene has been isolated and shown to encode a 1209 amino acid protein that shares no extended sequence similarity with known proteins. thr mRNA is present as maternal mRNA that degrades at the time of cellularization. At this and all subsequent times during embryogenesis, zygotic expression correlates with mitotic proliferation. These observations, together with the observation that the zygotic phenotype of thr mutant embryos is influenced by the maternal genotype, suggest that the embryonic phenotype results from exhaustion of the maternal thr contribution and does not reflect a developmentally restricted requirement for thr function. Our results indicate that the novel thr product is required specifically for chromosome disjunction during all mitoses.

Citron Kinase is an essential effector of the Pbl-activated Rho signalling pathway inDrosophila melanogaster

“…This pattern of expression, which is initially ubiquitous then restricted to the nervous system (Foe et al, 1993) is very similar to that observed for known cell cycle genes [e.g. three rows, D'Andrea et al (D'Andrea et al, 1993)], consistent with a role for citron in proliferation. The association of citron expression with proliferating tissues contrasts with the widespread expression of another Rho effector kinase rok, and of RhoA, the putative upstream activator of Citron, consistent with Rok and RhoA having roles beyond proliferation (Hariharan et al, 1995).…”

Epithelial re-organization and dynamics of progression through mitosis inDrosophilaseparase complex mutants

“…pim and thr are not required for centrosome duplication and cytokinesis Our previous phenotypic characterizations of fixed pim and thr mutant embryos had suggested that cytokinesis during mitosis 15 is not completed in these mutants (D'Andrea et al, 1993;Stratmann and Lehner, 1996). Cleavage furrows pinching the undivided chromosomes were often observed in fixed mutant embryos at the stage of mitosis 15.…”

“…After degradation of PIM at the metaphaseto-anaphase transition, SSE promotes sister chromatid separation and also cleaves the associated THR subunit, a process that contributes to timely SSE inactivation (Herzig et al, 2002). We have previously shown that a loss of PIM, THR or SSE function results in a failure of sister chromatid separation during mitosis (D'Andrea et al, 1993;Jäger et al, 2001;Stratmann and Lehner, 1996). In addition, non-cleavable THR versions were shown to interfere with cellularization (Herzig et al, 2002), which corresponds to a modified form of cytokinesis converting the syncytial into the cellularized blastoderm during interphase of cycle 14.…”

The transcription factor E2F is required for S phase during Drosophila embryogenesis.

“…Maternally supplied products are sufficient to support cell cycle pro- gression in the Drosophila embryo until G2 of cycle 14, when string gene expression is required. Whereas expression of additional cell cycle genes becomes essential as embryogenesis progresses, to date only five loci have been shown to be essential prior to cycle 17 (string, three rows, pimples, pebble, and the histone locus) (Edgar and O'Farrell 1989;Lehner 1992;D'Andrea et al 1993;Smith et al 1993). …”