Alternative mRNA splice variants in Drosophila 



JESO Volume 139, 2008 



transcripts analyzed a variant maintaining intron 1 while intron 2 was spliced out was 

 never observed. In other words splicing of intron 2 was predicated upon prior splicing of 

 intron 1. By contrast variants were identified with intron 3 spliced out while introns 1 and 

 2 were retained indicating that prior splicing of intron 1 is not a prerequisite for intron 3 

 removal. 



Discussion 



Some mammalian DNA and RNA viruses, lacking introns, such as Herpes 

 simplex virus can completely shut down host mRNA splicing machinery without affecting 

 viral gene expression (Muhlemann et al. 2000; Lindberg et al. 2002). On the other hand, 

 Vaccinia virus and Adenovirus rely on the host splicing machinery to process their genes 

 (Yue et al. 1999; Huang et al. 2002). Conversely, mammalian hosts have shown to use 

 alternative transcripts to combat viral infection (Dinesh-Kumar et al. 2000; Fridborg et al. 

 2004). 



Here we analyzed splice variant transcripts of the Drosophila gene Oggl 

 following FHV infection of macrophage-like Drosophila DL2 cells. FHV does not 

 itself undergo splicing, thus eliminating the possibility that the splicing machinery was 

 manipulated for viral infection. Drosophila DL-2 cell lines were utilized since they 

 exhibit macrophage-like genotypes, with receptors capable of detecting viral RNA and 

 initiating native immune response (Shields et al. 1975; Rehli et al. 2003). 



All Oggl splice variants identified in this study maintained some or all introns. 

 Since transcripts with un-excised introns were observed in the FHV infected as well as 

 in the mock-infected control cells it is unlikely that the transcripts represented infection- 

 related disregulation of splicing. These observations indicated that the transcripts detected 

 in Drosophila cells were not genuine alternative transcripts but rather immature transcripts 

 awaiting further splicing. Even though splicing is normally co-transcriptional, splicing 

 can occur post-transcriptionally (Aguilera 2005). In the control, non-infected cells, the 

 intron retention pattern of the splice variants (retention of intron 3 or introns 2 and 3 ) was 

 consistent with sequential removal of introns. However, in the FHV infected cells intron 

 removal was not sequential, with some transcripts retaining intron 2 or, introns 1 and 2. 

 Unspliced transcripts were also detected indicating that following infection splicing is not 

 co-transcriptional. 



Overall, splicing was adversely affected by FHV infection in Drosophila DL2 

 cells. Following FHV inoculation, we observed an accumulation of immature Oggl 

 transcripts suggesting a retardation of host gene splicing. This observation and the shift 

 in the types of transcripts observed during infection implicate changes in the regulation 

 or localization of some splicing factors as the likely mechanism responsible. These 

 changes were likely host-directed since the simple FHV lacks the molecular machinery to 

 implement changes in splicing. 



The results support the conclusion that viral infection may be accompanied 

 by host-produced splicing instability, which has the potential to generate novel splice 

 variants not associated with the generation of useful splice variants for either the host 

 or the infecting virus. Nonetheless, this instability has the potential of degrading the 



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