Identification of two processed pseudogenes of the human Tom20 gene

The open reading frame (ORF) of the human Tom20 gene (hTom20) was amplified by PCR from a HeLa cDNA library using primers based on the sequence of HUMRSC145 and cloned into a pET15b vector. Amplification of human genomic DNA using these primers yielded a DNA fragment of the same size as that of the ORF of hTom20 cDNA. Sequencing of this fragment revealed that: (1) it has the same number of base pairs as the ORF of hTom20 cDNA (438 bp); and (2) the two sequences differ by 14 single base pair substitutions (97% similarity) causing eight changes in the amino acid sequence and two premature stop codons. Further amplification of human genomic DNA adaptor-ligated libraries using primers based on HUMRSC145 revealed three different sequence-related genomic regions; one corresponding to the fragment referred above, another corresponding to the hTom20 gene, and a third fragment of which the sequence differs from the ORF of hTom20 cDNA by only 22 base pair substitutions and a deletion of 4 bp. We conclude that, in addition to the hTom20 gene, there are two genomic DNA sequences (Psi 1Tom20 and Psi 2Tom20) that are processed pseudogenes of hTom20. Aspects concerning their evolutionary origin are discussed.