Characterization of the mtTFA gene and identification of a processed pseudogene in rat

New isoforms of human mitochondrial transcription factor A detected in normal and tumoral cells

Novel alternatively spliced variants of the human mitochondrial transcription factor A predicted by the computational tool ASPic were experimentally validated in different normal and tumoral human tissues by RT-PCR and DNA sequencing. The comparison between the 5'UTR length and the distribution of the different transcripts showed that the transcripts with the shortest 5'UTR are present in all the investigated tissues, while the longest 5'UTR seems to be related to tissue-specificity. Studies about the localization and function of the most widely diffuse alternative isoform Tr6 were carried out.

Human mitochondrial transcription factor A possesses multiple subcellular targeting signals

The mitochondrial transcription factor A (TFAM) is a member of a high-mobility group (HMG) family represented mostly by nuclear proteins. Although nuclear localization of TFAM has been demonstrated in some tumors and after treatment of tumor cells with anticancer drugs, the significance of these observations has not been fully elucidated. Here we report that both TFAM overexpression and impairment of its mitochondrial targeting can result in nuclear accumulation of the protein. Both M1 and M7 methionines of human TFAM (hTFAM) can be used for translation initiation with almost equal efficiency resulting in two polypeptides. The shorter polypeptide, however, is not located in the nucleus, despite truncation in the mitochondrial targeting sequence, and both isoforms are targeted to mitochondria with similar efficiency. We further demonstrate that nuclear TFAM confers significant cytoprotection against the chemotherapeutic drugs etoposide, camptothecin, and cisplatin. Three regions of hTFAM [HMG-like domain 1 (HMG1) and HMG-like domain 2 (HMG2), as well as the tail region] can effect nuclear accumulation of enhanced green fluorescent protein (EGFP) fusions. The HMG1 domain contains a bipartite nuclear localization sequence whose identity is supported by site-directed mutagenesis. However, this bipartite nuclear localization sequence is weak, and both N-terminal and C-terminal flanking sequences enhance the nuclear targeting of EGFP. Finally, several mutations in the HMG1 domain increased the mitochondrial targeting of the EGFP fusions, suggesting that the mitochondrial targeting sequence of hTFAM may extend beyond the cleavable presequence.

History of the Tfam gene in primates

Tfam is a single copy nuclear gene mapping on chromosome 10 in human and mouse, 20 in rat and 12 in Presbytis cristata. It encodes for an HMG (high-mobility-group) protein showing a high affinity with the two transcriptional promoters and other mitochondrial DNA regions. It is an activator of mitochondrial transcription acting in the presence of mitochondrial RNA polymerase and of transcription factor B. Other interesting features of Tfam gene in human and rat are reported such as the existence of a smaller isoform, originated by an alternative splicing mechanism of the exon 5 (delta5 isoform) and the presence of different processed pseudogenes in addition to the active copy of the gene. In order to widen knowledge about Tfam gene and the appearance of some of its properties in the evolutionary history of primates, we have studied some aspects of this gene in different species. In particular we have determined its chromosomal localization, suggesting that its locus is highly conserved; we have searched for the presence of the delta5 isoform, demonstrating that it is present only in hominids; we have provided evidence of Tfam processed pseudogenes in the majority of the analysed genomes. Sequence data from this article have been deposited in the EMBL nucleotide database.

Study of the mitochondrial transcription factor A (Tfam) gene in the primate Presbytis cristata

The mitochondrial transcription factor A (Tfam) is a member of the HMG-box protein family, necessary for both transcription and maintenance of mitochondrial DNA. The gene is structured in seven exons and six introns and it is estimated to span about 10 kb in mouse, human and rat. In addition to the full length mRNA of Tfam, a shorter mRNA isoform lacking exon 5 has been found to be widely distributed in human and rat tissues. Here we present the isolation and characterization of Tfam gene in the primate Presbytis cristata which belongs to the Cercopithecidae family. We have determined the complete CDS sequence, the size of all the six introns, the complete sequences of the three shorter ones (I, III, VI) and the partial sequences of the long introns (II, IV, V). The comparison with other available Tfam sequences from mammals has revealed a high degree of conservation (above 90%) both in CDS and introns. By in situ hybridization (FISH) experiments we have mapped Tfam gene on chromosome 12 which, according to other cytogenetics studies, is the homologous region of chromosome 10, where human Tfam has been mapped. Moreover we have searched for the presence of alternatively spliced isoforms through several approaches, such as RT-PCR and differential hybridization. In Presbytis cristata we have not detected the presence of any spliced isoforms lacking exons; however we have identified one isoform in which part of the intron I is retained in the mRNA. The inclusion of this portion of intron I would originate an early stop codon if translated.

Vascular smooth muscle cell proliferation is dependent upon upregulation of mitochondrial transcription factor A (mtTFA) expression in injured rat carotid artery

Consistent with the physiological response to increased energy demand in proliferating cells, the number of mitochondria is upregulated in synthetic states of vascular smooth muscle cells (VSMC) in atherosclerotic lesion. We hypothesized that mitochondrial transcription factor A (mtTFA), a prerequisite factor for the transcription and replication of mtDNA, may be upregulated in VSMC of injured rat carotid artery, and that inhibition of its expression can attenuate the intimal thickening. Changes of intimal thickening and mtTFA expression by a treatment with antisense oligodeoxynucleotides (ODN) for mtTFA were investigated in balloon-injured rat carotid artery model. The expression of mtTFA was upregulated as early as 3 h up to 7 days after balloon injury. Delivery of ansisense ODN for mtTFA from adventitia side to injured arterial wall caused a significant decrease in intima-to-media (I/M) ratio. Furthermore, the increase in immunoreactivity and mRNA expression of mtTFA in injured artery as well as the number of mitochondria in intimal VSMC was abrogated by antisense ODN treatment. These data demonstrate that expression of mtTFA is upregulated in intimal VSMC of injured rat carotid artery, and that suppression of mtTFA expression by antisense ODN can attenuate intimal thickening after balloon injury.

Human mitochondrial transcription factor A (mtTFA): gene structure and characterization of related pseudogenes

Mitochondrial transcription factor A (mtTFA or Tfam) is a 25 kDa protein encoded by a nuclear gene and imported to mitochondria, where it functions as a key regulator of mammalian mitochondrial (mt) DNA transcription and replication. The coding sequence of the human mtTFA gene is reported in the literature and the sizes of few introns are known. In this paper we present the genomic structure of the human mtTFA gene along with the complete sequence of its six intronic regions. Three of the introns (I, III, VI) have been found to be less than 600 bp, while the other three were greater than 1.8 kb. In the course of this work, we discovered that, in addition to the active copy, different homologous sequences identified as processed pseudogenes psi h-mtTFA have been isolated and sequenced. Using an 'in silico' mapping approach we determined their locations on chromosomes 7, 11 and X. psi h-mtTFA locations are different from that of the gene, previously reported on chromosome 10. Transcription analysis by means of reverse transcriptase-polymerase chain reaction has shown that other than the RNA corresponding to the full-length transcript, an isoform lacking 96 bp is also present. Among the three sequenced pseudogenes only one of them located on chromosome 11 has been found to be transcribed in Jurkat cells under these culture conditions, even though transcription initiation and binding sites for different transcription factors have also been found upstream from the other two pseudogenes.