A Drosophila strain, carrying a P[lacW] element in the promoter of the protein phosphatase 2A (PP2A) catalytic subunit gene at chromosomal location 28D, has been identified using plasmid rescue of the P element and adjoining genomic DNA in Escherichia coli. Reversion mutagenesis was employed to demonstrate that the observed phenotype of the Drosophila strain was due to a single P[lacW] element insertion at 28D and to create three deficiency strains at this locus. Drosophila heterozygous for

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... cW]28D have reduced levels of PP2A mRNA and reduced PP2A catalytic activity against four different substrates compared to wild type, while homozygotes are deduced to have approximately 20% of wild-type PP2A activity. P[lacW]28D homozygotes, termed microtubule star (mts), die in embryo-genesis around the time of cellularisation, exhibiting over-condensed chromatin and a block in mitosis between prophase and the initiation of anaphase. Multiple centrosomes are visible in cellularised embryos, suggesting that PP2A may play a role in coupling the nuclear and centrosome cycles. When embryos arrest just prior to cellularisation, disorganised elongated arrays of microtubules radiate from centrosomes in all directions, but they are rarely associated with any DNA, suggesting that PP2A is required for the attachment of microtubules to chromosomal DNA at the kinetochore.