Unique structural characteristics and evolution of a cluster of venom phospholipase A2 isozyme genes of Protobothrops flavoviridis snake

Protobothrops flavoviridis (Crotalinae) venom gland phospholipase A 2 (PLA 2 ) isozyme genes have evolved in an accelerated manner to acquire diverse physiological activities in their products. For elucidation of the multiplication mechanism of PLA 2 genes, a 25,026 bp genome segment harboring five PLA 2 isozyme genes was obtained from Amami-Oshima P. flavoviridis liver and sequenced. The gene PfPLA 2 encoded [Lys 49 ]PLA 2 called BPII, the gene PfPLA 4 neurotoxic [Asp 49 ]PLA 2 called PLA-N, the gene PfPLA 5 basic [Asp 49 ]PLA 2 called PLA-B, and PfPLA 1(ψ) and PfPLA 3(ψ) were the inactivated genes. The 5′ truncated reverse transcriptase (RT) elements, whose intact forms constitute long interspersed nuclear elements (LINEs), were found in close proximity to the 3′ end of PLA 2 genes and named PLA 2 gene-coupled RT fragments (PcRTFs). The facts that PcRTFs have the stem–loop and repetitive sequence in the 3′ untranslated region (UTR) which is characteristic of CR1 LINEs suggest that PcRTFs are the debris of P. flavoviridis ancestral CR1 LINEs, denoted as Pf CR1s. Since the associated pairs of PLA 2 genes and PcRTFs are arranged in tandem in the 25,026 bp segment, it is thought that an ancestral PLA 2 gene- Pf CR1 unit ( PfPLA - Pf CR1) which was produced by retrotransposition of Pf CR1 by itself to the 3′ end of PLA 2 gene duplicated several times to form a multimer of PfPLA - Pf CR1, a cluster of PLA 2 genes, in the period after Crotalinae and Viperinae snakes branched off. Recombinational hot spot of a 37 bp segment, named Scomb , was found in the region 548 bp upstream from the TATA box of PLA 2 genes. Thus, it could be assumed that multiplication of PfPLA - Pf CR1 occurred by unequal crossing over of the segment, - Scomb - PfPLA - Pf CR1- Scomb- . The Pf CR1 moieties were afterward disrupted in the 5′ portion to PcRTFs. The detection of two types of PcRTFs different in length which were produced by elimination of two definitive sequences in Pf CR1 moiety possibly by gene conversion clearly supports such process but not multiplication of the PLA 2 gene-PcRTF unit.