BMC Genomics BioMed Central 3BR2M0e0Cs2eG,eanrocmhicsarticle x Complete genome sequence of a novel extrachromosomal virus-like element identified in planarian Girardia tigrina Denis V Rebrikov, Maria E Bulina, Ekaterina A Bogdanova, Loura L Vagner and Sergey A Lukyanov* Address: Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry RAS, Miklukho-Maklaya 16/10, 117871 Moscow, Russia E-mail: Denis V Rebrikov - den@ibch.ru; Maria E Bulina - biomasha@mail.ru; Ekaterina A Bogdanova - katya@ibch.ru; Loura L Vagner - vagner@evrogen.ru; Sergey A Lukyanov* - luk@ibch.ru *Corresponding author Published: 13 June 2002 BMC Genomics 2002, 3:15 Received: 3 December 2001 Accepted: 13 June 2002 This article is available from: http://www.biomedcentral.com/1471-2164/3/15 © 2002 Rebrikov et al; licensee BioMed Central Ltd. Verbatim copying and redistribution of this article are permitted in any medium for any purpose, pro- vided this notice is preserved along with the article's original URL. Abstract Background: Freshwater planarians are widely used as models for investigation of pattern formation and studies on genetic variation in populations. Despite extensive information on the biology and genetics of planaria, the occurrence and distribution of viruses in these animals remains an unexplored area of research. Results: Using a combination of Suppression Subtractive Hybridization (SSH) and Mirror Orientation Selection (MOS), we compared the genomes of two strains of freshwater planarian, Girardia tigrina. The novel extrachromosomal DNA-containing virus-like element denoted PEVE (Planarian Extrachromosomal Virus-like Element) was identified in one planarian strain. The PEVE genome (about 7.5 kb) consists of two unique regions (Ul and Us) flanked by inverted repeats. Sequence analyses reveal that PEVE comprises two helicase-like sequences in the genome, of which the first is a homolog of a circoviral replication initiator protein (Rep), and the second is similar to the papillomavirus E1 helicase domain. PEVE genome exists in at least two variant forms with different arrangements of single-stranded and double-stranded DNA stretches that correspond to the Us and Ul regions. Using PCR analysis and whole-mount in situ hybridization, we characterized PEVE distribution and expression in the planarian body. Conclusions: PEVE is the first viral element identified in free-living flatworms. This element differs from all known viruses and viral elements, and comprises two potential helicases that are homologous to proteins from distant viral phyla. PEVE is unevenly distributed in the worm body, and is detected in specific parenchyma cells. Background Freshwater planarians (Platyhelminthes, Turbellaria, Tri- cladida) are widely employed for pattern formation inves- tigations. planaria differ from other models used to study morphogenesis (insects, nematodes, fish, amphibians and mammals) by their phenomenal ability to regenerate [1]. Moreover, planaria are used for in vivo tumorogenicity studies of pure compounds and complex environmental mixtures, and for mechanistic studies of tumorogenesis [2]. Page 1 of 9 (page number not for citation purposes)