chapter  3
152 Pages

The Human Mitochondrial Genome: Mechanisms of Expression and Maintenance

Introduction ............................................................................ 97 General Features of Human mtDNA ................................. 100

Structure, Copy Number, and Organization ............. 100 Gene Content, Codon Usage, and Decoding

Mechanism ........................................................ 105 Transmission ............................................................... 109

Transmission of the Mitochondrial Genome .. 109 Transmission of mtDNA Sequence Variants.. 110

Transcription of mtDNA...................................................... 113 Initiation of Transcription .......................................... 113

cis-Acting Elements.......................................... 113 trans-Acting Factors ......................................... 115

Elongation and Termination of Transcription .......... 120

Processing of Primary Transcripts ............................ 123 Post-Transcriptional Modifications ............................ 130

Modifications of tRNA Molecules .................... 130 Modifications of rRNA and mRNA

Molecules ............................................. 134 Mitochondrial Protein Synthesis ........................................ 135

Mitochondrial tRNAs .................................................. 135 Mitochondrial Ribosomes............................................ 137 Initiation of Translation ............................................. 141 Elongation of Translation ........................................... 146 Termination of Translation......................................... 148

Replication of mtDNA.......................................................... 149 General Features of Replication ................................ 149 The Strand-Asynchronous Model of Replication ...... 151

Basic Mechanism.............................................. 151 Initiation of H-Strand Synthesis..................... 153 Initiation of L-Strand Synthesis ..................... 155

The Strand-Synchronous Model of Replication ........ 156 Maturation of the Daughter mtDNA Molecules ....... 159 trans-Acting Factors Involved in Replication............ 160

Mitochondrial RNases ...................................... 160 DNA Polymerase γ............................................ 163 Mitochondrial Single-Stranded

Binding Protein................................... 167 Mitochondrial DNA Helicase and Possible

Primase................................................ 168 Mitochondrial DNA Topoisomerases ............... 170 Mitochondrial DNA Ligase .............................. 173

Control of mtDNA Copy Number ....................................... 175 Variations in mtDNA Copy Numbers........................ 175 Transcriptional Control of Nuclear Genes

Encoding Mitochondrial Factors...................... 177 Control of mtDNA Copy Number by cis-and

trans-Acting Factors ......................................... 179 Multiple Roles for TFAM............................................ 181

Concluding Remarks............................................................ 185 References............................................................................. 186

INTRODUCTION

Mitochondria are essential eukaryotic organelles whose principal function is to generate ATP through the process of oxidative phosphorylation (1). They also house many additional metabolic pathways and are involved in Ca2+ homeostasis, apoptosis, and thermogenesis (2-4). Mitochondria possess two membranes: an outer membrane, which encloses the organelle and separates it from the cytosol, and a larger inner membrane. The latter can be further divided into two contiguous components: the inner boundary membrane that apposes the outer membrane, and the cristae membrane that projects into the matrix compartment (5,6). The five multisubunit complexes (I through V) of the oxidative phosphorylation system required for ATP synthesis are embedded in the cristae membrane (7).