The AT-Hook Protein D1 Is Essential for Drosophila melanogaster Development and Is Implicated in Position-Effect Variegation

Abstract

We have analyzed the expression pattern of the D1 gene and the localization of its product, the AT hook-bearing nonhistone chromosomal protein D1, during Drosophila melanogaster development. D1 mRNAs and protein are maternally contributed, and the protein localizes to discrete foci on the chromosomes of early embryos. These foci correspond to 1.672- and 1.688-g/cm³ AT-rich satellite repeats found in the centromeric heterochromatin of the X and Y chromosomes and on chromosomes 3 and 4. D1 mRNA levels subsequently decrease throughout later development, followed by the accumulation of the D1 protein in adult gonads, where two distributions of D1 can be correlated to different states of gene activity. We show that the EP473 mutation, a P-element insertion upstream of D1 coding sequences, affects the expression of the D1 gene and results in an embryonic homozygous lethal phenotype correlated with the depletion of D1 protein during embryogenesis. Remarkably, decreased levels of D1 mRNA and protein in heterozygous flies lead to the suppression of position-effect variegation (PEV) of the white gene in the white-mottled (w^m4h) X-chromosome inversion. Our results identify D1 as a DNA-binding protein of known sequence specificity implicated in PEV. D1 is the primary factor that binds the centromeric 1.688-g/cm³ satellite repeats which are likely involved in white-mottled variegation. We propose that the AT-hook D1 protein nucleates heterochromatin assembly by recruiting specialized transcriptional repressors and/or proteins involved in chromosome condensation.

We thank Leonora Poljak, François Payre, and Michèle Crozatier for discussions and Dale Dorsett and Joel Eissenberg for comments on the manuscript. We are grateful to our colleagues at the Centre de Biologie du Développement (CBD, Toulouse, France) for advice and use of Drosophila facilities and to Eric Drier (Cold Spring Harbor Laboratory) for help in quantitative assays of PEV. We also thank David Villa for help in the preparation of the figures, Joshua Käs for expert technical assistance, and Ezra Käs for a critical reading of the manuscript. We are particularly grateful to the Drosophila Stock Centre and to Peter Maroy, Department of Genetics and Molecular Biology, Szeged, Hungary, for maintaining and distributing fly stocks. We also thank the referees of the manuscript for constructive criticism and useful suggestions.

This work was supported by grants from the Centre National de la Recherche Scientifique (CNRS), the Ligue Nationale Contre le Cancer and the Association pour la Recherche sur le Cancer (ARC, grants no. 9284 and 5698). N. Aulner and D. Jullien gratefully acknowledge graduate fellowships from the ARC.