The Drosophila cellularization gene nullo produces a blastoderm-specific transcript whose levels respond to the nucleocytoplasmic ratio.

Abstract

The initial development of the Drosophila embryo is characterized by rapid nuclear mitosis without cytokinesis. After 13 such mitoses, a coordinated cell division process called cellularization occurs, during which membranes simultaneously enclose each nucleus in a cell. Cellularization requires the establishment of a hexagonal network of actin and myosin filaments in the cortex of the embryo; the filaments are located on the cytoplasmic face of the invaginating membrane furrows. Zygotic expression of the nullo gene is essential for the maintenance of an intact actin-myosin network. We have cloned the nullo gene and present its sequence as well as a characterization of nullo transcript levels in wild-type and mutant embryos. The nullo gene encodes a predicted protein of 213 amino acids, a large proportion of which is basic, hullo transcripts are first detectable at nuclear cell cycle 11, peak in accumulation at the end of cycle 13, and disappear rapidly as cellularization begins. The gene does not appear to be expressed at any other time in the life of the organism. The normal accumulation of nullo transcripts does not require gene activity of other zygotic cellularization genes. The regulation of hullo RNA levels during cycle 14, however, is coupled to the nucleocytoplasmic ratio, which also controls the cessation of rapid, synchronous mitosis just before cellularization.

Conserved Domains of the Nullo Protein Required for Cell-Surface Localization and Formation of Adherens Junctions

“…To study factors that influence basal junction formation, we examined the nullo mutant line LII27.32R5, in which 20% of nullo hemizygous males survive to adulthood at 25°C. The R5 mutation results from a rearrangement upstream of the nullo ORF and does not produce sufficient Nullo protein to detect by Western blot (Rose and Wieschaus, 1992; our unpublished results). Further analysis revealed that the genomic region required for viability of the hemizygous males mapped to the same site on the X-chromosome as nullo (our unpublished results).…”

“…Cloning of the nullo gene revealed little about potential mechanisms for its involvement in basal junction formation: nullo encodes a small, highly basic protein that lacks homology to any previously characterized proteins (Rose and Wieschaus, 1992). In this article we identify a homologue of Nullo from D. virilis and use interspecies rescue and deletion analysis to identify conserved regions of Nullo that are required for basal junction formation.…”

“…The nullo gene encodes a small novel protein, with no motifs or homologies to suggest a mechanism for its involvement in basal junction formation (Rose and Wieschaus, 1992). To identify functionally important regions of the Nullo protein, we cloned a nullo homologue from the related species D. virilis.…”

“…nullo mutants were obtained from the lines Df(1)6F1-2, Df(1)LVII9, Df(1)LV16 (16.3), Df(1)L-II-27-32 R5 (Rose and Wieschaus, 1992), or C(1)DX y w. Ore-R was used as the wild-type stock. The mat67.15 stock containing GAL4-VP16 under the control of the maternal ␣-tubulin promoter was a gift of D. St. Johnston.…”

“…A genomic D. virilis phage library (kindly provided by P. Schedl) was plated and screened with a 32 P-labled D. melanogaster nullo cDNA clone (Rose and Wieschaus, 1992), using low-stringency hybridization conditions (42°C, 29% formamide), as in O'Neil and Belote (1992). A 4.5-kb SacI fragment from one of two phage clones that produced a signal through repeated rounds of hybridization was subcloned into the pBluescript (pBS; Invitrogen, Carlsbad, CA) plasmid vector and sequenced.…”