Cloning and sequence analysis of the β2-microglobulin transcript from flounder, Paralichthys olivaceous

β2-Microglobulin (β2M) is a protein found free-form in the serum or on the cell surface non-covalently associated with the α-chain of the class I major histocompatibility (MHC-I) complex. The full-length cDNA containing β2M was cloned from flounder, Paralichthys olivaceous. The transcript consists of 1610 nucleotides (nts), including an open reading frame (ORF) of 384 nts encoding a polypeptide of 128 amino acids. The amino acid sequence of β2M in flounder is 59, 57, 56, and 48% conserved in catfish, rainbow trout, zebrafish, and humans, respectively. Genomic Southern hybridization suggested the presence of a single copy of β2M in the flounder genome, and reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis detected the β2M transcript in the head kidney, spleen, body kidney, liver, and muscle tissues of the flounder. PCR amplification and sequence analysis revealed the lack of an intron in the β2M gene. The phylogenetic analysis confirmed the evolutionary diversion of the β2M protein among warm-blooded vertebrates and fish, and the separation between freshwater and seawater fish.