APOLIPOPROTEIN-B RNA SEQUENCE-3' OF THE MOORING SEQUENCE AND CELLULARSOURCES OF AUXILIARY FACTORS DETERMINE THE LOCATION AND EXTENT OF PROMISCUOUS EDITING

Apolipoprotein B (apoB) RNA editing involves a cytidine to uridine tra nsition at nucleotide 6666 (C6666) 5' of an essential cis-acting 11 nu cleotide motif known as the mooring sequence. APOBEC-1 (apoB editing c atalytic sub-unit 1)serves as the site-specific cytidine deaminase in the context of a multiprotein assembly, the editosome, Experimental ov er-expression of APOBEC-1 resulted in an increased proportion of apoB mRNAs edited at C6666, as well as editing of sites that would otherwis e not be recognized (promiscuous editing). In the rat hepatoma McArdle cell line, these sites occurred predominantly 5' of the mooring seque nce on either rat or human apoB mRNA expressed from transfected cDNA, In comparison, over-expression of APOBEC-1 in HepG2 (HepG2-APOBEC) hum an hepatoma cells, induced promiscuous editing primarily 5' of the moo ring sequence, but sites 3' of the C6666 were also used more efficient ly. The capacity for promiscuous editing was common to rat, rabbit and human sources of APOBEC-1, The data suggested that differences in the distribution of promiscuous editing sites and in the efficiency of th eir utilization may reflect cell-type-specific differences in auxiliar y proteins. Deletion of the mooring sequence abolished editing at the wild type site and markedly reduced, but did not eliminate, promiscuou s editing. In contrast, deletion of a pair of tandem UGAU motifs 3' of the mooring sequence in human apoB mRNA selectively reduced promiscuo us editing, leaving the efficiency of editing at the wild type site es sentially unaffected, ApoB RNA constructs and naturally occurring mRNA s such as NAT-1 (novel APOBEC-1 target-1) that lack this downstream el ement were not promiscuously edited in McArdle or HepG2 cells, These f indings underscore the importance of RNA sequences and the cellular co ntext of auxiliary factors in regulating editing site utilization.