The Rat Serum Albumin Gene

The messenger RNA's that encode the Proteins rat serum albumin (RSA) and rat alphafetoprotein (RAFP) have been purified to virtual homogeneity by a combination of immunoprecipitation of polysomes and other physical isolation methods. Radioactive cDNA copies of these mRNA's have been prepared and used to monitor the changes in abundance of RSA- and RAFP-synthesizing polysomes in neonatal rat liver and in Morris hepatoms 7777, a rat liver tumor. These cDNA probes have also been used to determine the stability and reiteration frequency of their respective genes in various rat tissues. The RSA mRNA sequence has been converted into a series of bacterial plasmids by recombinant DNA methodology, and the RSA gene has been isolated from a library of recombinant bateriophage. Restriction endonuclease site mapping, R-loop mapping, "Southern" blot and extensive nucleotide sequence determination have been employed to elucidate the organization of the cloned sequences. The rat serum albumin gene has been found to be interrupted at fourteen locations by introns. The fifteen exons are spread over approximately 15,000 nucleotides of contiguous chromosomal DNA. The evolutionary history of albumin has been deduced by analysis of the patterns of internal periodic homology in this gene. Albumin apparently evolved by a series of at least three intragenic duplications followed by accumulation of many point mutations and small deletions. These events probably occurred over 300 million years ago. The rat alphafetoprotein gene has been shown to be related to the rat serum albumin gene by a common ancestor gene, and thus two (or possibly more) highly complex genes with many exons and many protein domains have evolved by duplication mechanisms. That this might be an important general source of the complexity of eukaryotic genomes is discussed.