Identification and developmental expression of a 5 '-3 ' exoribonuclease from Drosophila melanogaster

In multicellular organisms, very little is known about the role of mRNA sta bility in development, and few proteins involved in degradation pathways ha ve been characterized. We have identified the Drosophila homologue of XRN1, which is the major cytoplasmic 5'-3' exoribonuclease in Saccharomyces cere visiae. The protein sequence of this homologue (pacman) has 59% identity to S. cerevisiae XRN1 and 67% identity to the mouse homologue (mXRN1p) in cer tain regions. Sequencing of this cDNA revealed that it includes a trinucleo tide repeat (CAG)(9) which encodes polyglutamine. By directly measuring pac man exoribonuclease activity in yeast, we demonstrate that pacman can compl ement the yeast XRN1 mutation. Northern blots show a single transcript of a pproximately 5.2 kb which is abundant only in 0-8-h embryos and in adult ma les and females. In situ hybridization analysis revealed that the pcm trans cripts are maternally derived, and are expressed at high levels in nurse ce lls. During early embryonic syncytial nuclear divisions, pcm transcripts ar e homogenously distributed. pcm mRNA is expressed abundantly and ubiquitous ly throughout the embryo during gastrulation, with high levels in the germ band and head structures. After germ band retraction, pcm transcripts are p resent at much lower levels: in agreement with the Northern results. Our ex periments provide the first example of an exoribonuclease which is differen tially expressed throughout development. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.