Baboon apolipoprotein C-I: cDNA and gene structure and evolution.

We have isolated and characterized cDNA and genomic clones for apolipoprotein C-I (apoC-I) encoded by the APOC1 locus in baboons. Baboon apoC-I cDNA is only 410 bp in size, but the gene spans 4.5 kb including four small exons and three introns containing a large number of Alu repeats. The coding sequences of apoC-I cDNA and genomic clones are identical, indicating that this genomic clone contains the functional gene for apoC-I rather than a pseudogene like human APOC1'. We also detected a second gene in Southern blots of baboon genomic DNA that may correspond to the human APOC1' pseudogene. Two start sites for baboon APOC1 transcription were mapped to nucleotides that are 7 and 9 bp downstream from the predominant start site for human APOC1 transcription. Alignment of Alu repeats showed that the 5' region of the baboon APOC1 gene is more similar to that of the human pseudogene APOC1', and the 3' region and coding sequences are more similar to those of human APOC1. These regions are separated by an Alu repeat that is present only in the baboon gene, perhaps reflecting its role in gene rearrangement or conversion. Sequence comparisons from baboon, human, dog, and rat showed extensive differences in apoC-I amino acid sequences, which are less conserved than nucleotide sequences. However, comparisons of hydrophilicity profiles show significant conservation of protein domains that may be important for apoC-I function.