DOWN-REGULATION OF MITOCHONDRIAL TRANSCRIPTION FACTOR-A DURING SPERMATOGENESIS IN HUMANS

Mitochondrial transcription factor A (mtTFA) is a key activator of mit ochondrial transcription in mammals, It also has a role in mitochondri al DNA (mtDNA) replication, since transcription generates an RNA prime r necessary for initiation of mtDNA replication, In the mouse, testis- specific mtTFA transcripts encode a protein isoform that is imported t o the nucleus rather than into mitochondria of spermatocytes and elong ating spermatids, We now report molecular characterization of human mt TFA (h-mtTFA) expression in somatic tissues and male germ cells. Simil arly to the mouse, analysis of cDNAs and Northern blots identified abu ndant testis-specific transcript isoforms generated by use of alternat e transcription initiation sites, However, unlike the mouse, none of t he testis-specific transcripts predicts a nuclear protein isoform, and Western blot analysis identified only the mitochondrial form of h-mtT FA in human testis, Immunohistochemistry and in situ hybridizations we re used to compare the distribution of mtTFA protein, testis-specific mtTFA transcripts, mtDNA and mtRNA in sections of human testis, Our re sults show that mtTFA protein and mtDNA exhibit parallel gradients wit h high levels in undifferentiated male germ cells and low levels or an absence in differentiated male germ cells, Testis-specific transcript s exhibit the opposite pattern, suggesting that in both humans and mic e, these testis-specific mtTFA transcripts down-regulate mtTFA protein levels in mammalian mitochondria. Our findings demonstrate that mtTFA does not have a critical role in the nucleus, suggest a mechanism for reducing mtDNA copy number number during spermatogenesis and have imp lications for the understanding of maternal transmission of mtDNA.