Cloning and expression of the AID gene in the channel catfish

A full-length activation-induced cytidine deaminase (AID or Aicda) cDNA has been obtained from the channel catfish (Ictalurus punctatus). A single open reading frame predicts a 209 amino acid protein that has 57% identity and 73% similarity with the AID proteins of mouse and human. All residues that have previously been found to be critical for deamination, as well as for somatic hypermutation, are conserved in the catfish AID. These residues are also conserved in AID proteins predicted, from genome database sequences, to be expressed in Fugu and zebrafish. The catfish AID is expressed at low levels in spleen, kidney, intestine and fin margins, but not in muscle, liver or brain. Immunoglobulin heavy chain (IgH) is also expressed in the tissues where AID is expressed. The 'ectopic' expression of AID in non-lymphoid tissue was unexpected and not readily explained. However, the identification of a fish AID gene will allow us to determine the tissue architecture and locations for affinity maturation in fish.