EVOLUTIONARY ORIGINS OF THE MAMMALIAN APOLIPOPROTEINB RNA EDITING ENZYME, APOBEC-1 - STRUCTURAL HOMOLOGY INFERRED FROM ANALYSIS OF A CLONEDCHICKEN SMALL-INTESTINAL CYTIDINE DEAMINASE

Mammalian apolipoproteinB (apoB) RNA editing is a site-specific deamin ation reaction that mediates the C to U conversion responsible for apo B48 production in the mammalian small intestine. This process is not d etected in chicken apoB RNA. Mammalian apoB RNA editing is mediated by a multicomponent enzyme complex that includes a single catalytic subu nit, apobec-1. In order to examine the evolution of apobec-1, we have cloned and characterized an orthologous cytidine deaminase cDNA isolat ed from chicken small intestine. Northern blot analysis revealed expre ssion restricted to the small intestine, colon and lung but not the li ver or other tissues. The cDNA encodes a single 31 kDa protein with fe atures reminiscent of other cytidine deaminases and with approximately 39% overall homology to rat apobec-1. The recombinant protein is a cy tidine deaminase with activity on a monomeric substrate that was found to be zinc-dependent. However, no RNA editing activity was detectable towards cytidine nucleotides presented in the context of an optimally configured mammalian apoB RNA template. These studies provide informa tion concerning the evolution of the apoB RNA editing machinery and in dicate that a chicken small intestinal cytidine deaminase with homolog y to apobec-1 demonstrates no activity on an RNA substrate.