Requirement for highly efficient pre-mRNA splicing during Drosophila early embryonic development

Abstract

Drosophila syncytial nuclear divisions limit transcription unit size of early zygotic genes. As mitosis inhibits not only transcription, but also pre-mRNA splicing, we reasoned that constraints on splicing were likely to exist in the early embryo, being splicing avoidance a possible explanation why most early zygotic genes are intronless. We isolated two mutant alleles for a subunit of the NTC/Prp19 complexes, which specifically impaired pre-mRNA splicing of early zygotic but not maternally encoded transcripts. We hypothesized that the requirements for pre-mRNA splicing efficiency were likely to vary during development. Ectopic maternal expression of an early zygotic pre-mRNA was sufficient to suppress its splicing defects in the mutant background. Furthermore, a small early zygotic transcript with multiple introns was poorly spliced in wild-type embryos. Our findings demonstrate for the first time the existence of a developmental pre-requisite for highly efficient splicing during Drosophila early embryonic development and suggest in highly proliferative tissues a need for coordination between cell cycle and gene architecture to ensure correct gene expression and avoid abnormally processed transcripts.When a fertilized egg develops into an embryo, the expression of many genes must be carefully timed and coordinated. Researchers regularly use a type of fruit fly called Drosophila to study development because it is small, it has a short lifespan, and its whole genome sequence is already known. The development of a Drosophila embryo starts with the nucleus of the fertilized egg, which contains most of the cell’s genetic material, dividing 13 times in quick succession, without the cell itself splitting. These divisions are amongst the fastest known for any animal, and given the fast developmental speed, the embryo must efficiently express all genes it needs to stay alive. Because cell division is known to inhibit gene expression this raises an interesting conundrum about the way fast cell proliferation and gene expression are coordinated.The first step of gene expression involves a length of DNA being transcribed to produce an intermediate molecule called a messenger RNA (mRNA), which is then translated to produce a protein. However, some mRNA molecules contain regions called ‘introns’ that are not translated and must instead be removed via a time-consuming process called ‘splicing’ before the protein is produced.At first a Drosophila embryo uses mRNA molecules that were spliced and packaged inside the egg by the mother, but later it starts to make its own mRNA molecules. The very first mRNA molecules made by the early embryo tend to be short and lack introns. The shortness of these molecules is thought to reflect the fact there is not enough time to produce longer mRNA molecules. Is the same ‘need for speed’ also responsible for the lack of introns in these molecules?Now, Guilgur et al. have tested this hypothesis by manipulating a gene named fandango, which codes for part of the cellular machinery that removes introns from mRNA molecules, in fruit flies. These mutant fruit flies had less of the Fandango protein than wild-type flies and while they passed through the early stages of development normally, they later developed defects—such as abnormally shaped cells. Guilgur et al. revealed that fandango mutants fail to splice out the introns in the mRNA molecules that are made in the early embryo, whereas similar mRNA molecules from the mother were spliced as normal. Further experiments suggested that wild-type embryos struggled to correctly splice an untypical early gene that had multiple introns.Together the findings of Guilgur et al. suggest that when nuclei (or cells) are dividing rapidly, there is a strong selective pressure to splice mRNA molecules quickly in the short time between the divisions. Furthermore, this pressure appears to have shaped the architecture of the earliest genes expressed in the Drosophila embryo, which is why the first mRNA molecules produced by the embryo itself tend not to contain introns.