Cloning of the kininogen gene from Lampetra japonica provides insights into its phylogeny in vertebrates

Kininogens, the precursors of bradykinins, ubiquitously exist in vertebrates, including mammals, birds, amphibians, and fishes. To elucidate the phylogeny of kininogen genes in early vertebrates, we cloned the full-length cDNA of kininogen gene from the liver of Lampetra japonica. The open reading frame of this sequence contained 546 bp and encoded 181 amino acids, including a cystatin domain without the canonical binding site for cysteine proteinases and a bradykinin domain. Our results suggested that in lampreys and most of other vertebrates, there might be only one kininogen gene, which was fused by certain sequences during vertebrate evolution and encoded proteins with more functions; however, another special kininogen gene, only encoding the bradykinin domain with multiple copies in some species, arose only in amphibians for adapting themselves to the unique environment. Using reverse transcription PCR, kininogen mRNA was also detected in lamprey gut, kidney, and leukocyte, but absent in lamprey buccal gland. Our findings may provide insights into the phylogeny of kininogen genes as well as other gene families in vertebrates.