Abstract
It is generally accepted that eukaryotic cell arises from prokaryotic cell, which means that organelle can be formed in prokaryotic cell. However, no such an instance has been detected till now. Here, we report organelle biogenesis in the endosymbiotic cyanobacterium TDX16 released from the green alga Haematococcus pluvialis, which occurred through six steps. (1) An inner intracytoplasmic membrane (IIM), an outer intracytoplasmic membrane (OIM) and an intervening peptidoglycan-like layer (PL) were synthesized by merging cytoplasmic membrane (CM)-derived thick margin vesicles, which partitioned the thylakoid-less cytoplasm into three compartments: an inner cytoplasm (ICP), an outer cytoplasm (OCP) and a sandwiched intracytoplasmic space (IS). (2) Osmiophilic granules that blistered from CM, OIM and IIM developed into primary thylakoids (PT) in ICP; while OCP disappeared and thus OIM and CM combined into a double-membraned cytoplasmic envelope (CE). (3) ICP decondensed; IIM and PT disassembled into tiny vesicles (TV) and double-membraned vesicles (DMV) respectively. Such that DNA fibers (DF) aggregated and migrated to PL. (4) TV fused into a double-membraned intracytoplasmic envelope (ICE), which re-compartmentalized the coalesced IS and ICP into a new intracytoplasmic space (NS) sequestering most DF and a new inner cytoplasm (NIC) with only few DF. Then ribosomes were formed in both NS and NIC, while DMV opened and extended into secondary thylakoids (ST) only in NIC. (5) NIC developed into the primitive chloroplast (PC) surrounded by ICE, in which ST disassembled, while ST-derived plastoglobuli developed into primitive eukaryotic thylakoids (PMT). After PL dismantled, the matrix of NS was concentrated and encased with the membranes synthesized from ICE-derived dotted vesicles into the primitive nucleus (PN). So, NS vanished, CE wrapped PC and PN. Outside CE, eukaryotic cell wall was formed by assembling sheath at the outer membrane of original cell wall and modifying the peptidoglycan layer. (6) Eukaryotic cytoplasm was built up from the matrix extruded from PN. Mitochondria were assembled in and segregated from PC by encapsulating a portion of stroma with the membranes synthesized from PMT-derived dense-margined vesicles. Then, most mitochondria turned into double-membraned vacuoles after matrix degradation, which mediated unconventional exocytosis and endocytosis. When this process finished, PN got matured into a nucleus enclosed by two sets of envelopes; PC matured into a chloroplast with its PMT maturing into thylakoids. Consequently, the prokaryotic TDX16 cell developed into a eukaryotic cell (TDX16-DE). Results of pigment analysis, 16S rRNA and genome sequencing revealed that TDX16 is a phycocyanin-containing cyanobacterium resembling Chroococcidiopsis thermali, which had acquired 10301 genes form its host, while TDX16-DE is a green alga, whose chloroplast contains chlorophyll b and lutein showing high similarity to that of Chlorella vulgaris. Therefore, the mechanism underlying organelle biogenesis in TDX16 was the integration and expression of the obtained genes. Taken together, these results demonstrated that an endosymbiotic prokaryote can develop into a new eukaryote by acquisition and recombination of its eukaryotic host’s DNA, which has profound effects on biology, particularly cell and evolutionary biology.
List of abbreviations
- AUG
- Autosporangium
- EW
- Eukaryotic cell wall
- C
- Chloroplast
- FM
- Fibrillary materials
- CD
- Chloroplast debris
- GA
- Golgi apparatus
- CE
- Cytoplasmic envelope
- GP
- Globular particles
- CF
- Chromatin fibers
- HGB
- Heterogenous globular bodies
- CG
- Cyanophycin granules
- IB
- Intranuclear body
- CHE
- Chloroplast envelope
- ICE
- Intracytoplasmic envelope
- CLM
- Cloudlike materials
- ICP
- Inner cytoplasm
- CM
- Cytoplasmic membrane
- IES
- Inter envelope space
- CPV
- Compound vesicles
- IIM
- Inner intracytoplasmic membrane
- CR
- Cristae
- IIS
- Inner intracytoplasmic space
- CV
- Combined vesicles
- INS
- Interspace
- CW
- Cell wall
- IS
- Intracytoplasmic space
- CX
- Carboxysomes
- ITB
- Internal body
- DF
- DNA Fibers
- IV
- Internal vesicle
- DGV
- Dense-margined vesicles
- IVS
- Invaginated space
- DLF
- DNA-like fibrils
- LD
- Lipid droplet
- DMF
- Double-layered membrane fragment
- LDB
- Less electron-dense bodies
- DMS
- Double-layered membrane segment
- LDM
- Less electron-dense materials
- DMV
- Double-membraned vesicles
- LM
- Limiting membrane
- DRV
- Dilated ring-shaped vesicles
- M
- Mitochondrion
- DSV
- Dense vesicle
- ME
- Mitochondrial envelope
- DT
- DNA threads
- MF
- Membrane fragments
- DV
- Dotted vesicles
- ML
- Microfibrils
- ED
- Electron-dense debris
- MLB
- Multilamellar body
- EDV
- Electron-dense vesicles
- MR
- Margin residues
- EF
- Electron-dense fibrils
- MS
- Membrane segments
- EG
- Electron-dense granules
- MT
- Membranous elements
- EIS
- Empty Inner space
- MV
- Microvesicles
- EL
- Electron-dense layer
- N
- Nucleus
- ELM
- Electron-translucent materials
- NE
- Nuclear envelope
- ELV
- Electron-translucent vesicles
- NIC
- New inner cytoplasm
- EM
- Eukaryotic cytoplasm
- NIS
- New inner intracytoplasmic space
- EOB
- Electron-opaque bodies
- NS
- New intracytoplasmic space
- EOP
- Electron-opaque particles
- NT
- Nucleoid-like structure
- EOM
- Electron-opaque materials
- NU
- Nucleoid
- EOV
- Electron-opaque vesicles
- NX
- New intracytoplasmic matrix
- EP
- Electron-dense particles
- OCP
- Outer cytoplasm
- EPM
- Electron-transparent materials
- OE
- Outer nuclear envelope
- ER
- Endoplasmic reticulum
- OG
- Osmiophilic granules
- ES
- Extracytoplasmic space
- OIM
- Outer intracytoplasmic membrane
- EV
- Electron-transparent vesicle
- OIS
- Outer intracytoplasmic space
- OPV
- Opaque-periphery vesicle
- SA
- Sporangium
- OM
- Outer membrane
- SG
- Starch granules
- OV
- Oblong vesicles
- SH
- Sheath
- P
- Peptidoglycan layer
- SMV
- Smaller vesicles
- PB
- Polyphosphate bodies
- SM
- Stroma
- PC
- Primitive chloroplast
- SOV
- Small opaque vesicle
- PCB
- Phycobilisomes
- SP
- Starch plate
- PD
- Pyrenoids
- ST
- Secondary thylakoids
- PG
- Plastoglobuli
- SV
- Small vesicles
- PL
- Peptidoglycan-like layer
- T
- Thylakoids
- PMT
- Primitive thylakoids
- TE
- Tubules
- PN
- Primitive nucleus
- TL
- Thylakoid-like structure
- PNE
- Primitive nuclear envelope
- TMF
- Two-layered membrane fragment
- PO
- Pores
- TMV
- Thick margin vesicle
- PT
- Primary thylakoids
- TV
- Tiny vesicles
- RB
- Ribosomes
- V
- Vacuole
- RM
- Residual membranes
- VB
- Vesicle-containing body
- RV
- Ring-shaped vesicles