Initial steps in RNA processing and ribosome assembly occur at mitochondrial DNA nucleoids

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Mammalian mitochondrial DNA (mtDNA) is organized into nucleoids where replication and transcription occur.
• Unlike bacteria and eukaryotic nucleoli, mtDNA does not have dedicated rRNA transcription units.
• The assembly process of mitochondrial ribosomes (mitoribosomes) is not well understood.

Purpose of the Study:

• To investigate the role of mtDNA nucleoids in RNA processing and mitoribosome assembly.
• To identify proteins involved in the initiation of mitochondrial ribosome assembly at the nucleoid.

Main Methods:

• Utilized SILAC (Stable Isotope Labeling by Amino acids in Cell culture) pulse-chase labeling experiments.
• Fractionated cellular components to isolate mtDNA nucleoids.
• Analyzed protein association with nucleoids and their labeling patterns over time.

Main Results:

• Mitochondrial RNA processing enzymes, including ribonuclease P (RNase P) and ELAC2, associate with mtDNA nucleoids.
• A subset of nascent mitochondrial ribosomal proteins (MRPs) also binds to nucleoids.
• SILAC labeling confirmed that nascent MRPs are recruited to nucleoids in a transcription-dependent manner and are dynamically associated.

Conclusions:

• The mtDNA nucleoid serves as a crucial hub for initiating mitochondrial ribosome assembly.
• Mitochondrial RNA processing enzymes and nascent MRPs cooperate at the nucleoid for efficient mitoribosome biogenesis.
• These findings establish the mtDNA nucleoid as a central control point for mitochondrial biogenesis.