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PRMT8 is a neuron-specific protein arginine methyltransferase in vertebrates. From data mining, we found a novel prmt8e6+43 splicing variant with a 43-nucleotide (nt) extension at the 5' of exon 6 in chicken. RT-PCR analyses confirmed the existence of two splicing variants but also detected a third upper signal. The triplet pattern detected in chicken suggests that one strand from the prmt8e6+43 transcript and one strand from the regular splicing products form a heteroduplex with a bulb conformation and the two transcripts are of similar abundance. One short plus one faint upper heteroduplex signal detected in mouse and human indicate that the level of the variant is much less than the normal one in mammals. The relative expression of the normal and prmt8e6+43 variants in different species can be inferred from the reads of intron 5 that contains the 43-nt extension or not in the RNA-seq data of NCBI Gene database. The results of the analyses showed that the prmt8e6+43 variant is relatively abundant in birds but much less or even not detected in mammalian species. As conserved intron 5 sequences and evidences of alternative splicing (AS) are detected in elephant shark, a cartilaginous fish with the slowest-evolving genome, we propose that the prmt8e6+43 variant is present in the common ancestor of jawed vertebrates. The prmt8e6+43 variant includes a premature termination codon and thus should encode a truncated PRMT8 with deletion from the dimerization arm. Western blot analyses showed very weak low-molecular-weight signals in chicken, which might be the C-terminal truncated PRMT8. Why avian species maintain high RNA but not protein levels of the prmt8e6+43 variant and whether the evolutionary conserved sequence and AS might regulate PRMT8 expression require further investigation., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Nine protein arginine methyltransferases (PRMTs) are conserved in mammals and fish. Among these, PRMT1 is the major type I PRMT for asymmetric dimethylarginine (ADMA) formation and is the most conserved and widely distributed one. Two chicken prmt1 splicing variants were assembled and confirmed by RT-PCR experiments. However, only two scaffolds containing single separate prmt1 exon with high GC contents are present in the current chicken genome assembly. Besides, prmt1 exons are scattered in separate small scaffolds in most avian species. Complete prmt1 gene has only been predicted from two falcon species with few neighboring genes. Crocodilians are considered close to the common ancestor shared by crocodilians and birds. The gene arrangements around prmt1 in American alligator are different from that in birds but are largely conserved in human. Orthologues of genes in a large segment of human chromosomal 19 around PRMT1 are missing or not assigned to the current chicken chromosomes. In comparison, prmt8, the prmt1 paralogue, is on chicken chromosome 1 with the gene arrangements downstream of prmt8 highly conserved in birds, crocodilians, and human. However, the ones upstream vary greatly in birds. Biochemically, we found that though prmt1 transcripts were detected, limited or none PRMT1 protein was present in chicken tissues. Moreover, a much higher level of PRMT8 protein was detected in chicken brain than in mouse brain. While PRMT8 is brain specific in other vertebrate species studied, low level of PRMT8 was present in chicken but not mouse liver and muscle. We also showed that the ADMA level in chicken was similar to that in mouse. This study provides the critical information of chicken PRMT1 and PRMT8 for future analyses of the function of protein arginine methyltransferases in birds.