I just read a fascinating “hypothesis” in the latest issue of Nature entitled Introns and the origin of nucleus cytosol compartmentalization.
The greatest divide in the living world exists between prokaryotes and eukaryotes (yes I know, there are viruses … but lets not get off topic!).
Generally, prokaryotes are devoid of membrane-bound organelles (including the nucleus, mitochondria, the endoplasmic reticulum), and their cytoskeletal systems are quite simple. These critters’ genes do not have introns (aka junk DNA) and their genome does not undergo much recombination. If prokaryotes had a strategy, it would be “multiply faster than your neighbors”. We call these critters bacteria and divide them into two groups: archeabacteria (includes many extreme bacteria) and eubacteria (common bacteria).
Eukaryotes on the other hand are complicated machines. They have many organelles that are each specialized to perfom a subset of the cellular functions. In addition most eukaryotes share the same extensive cytoskeletal toolkit. Eukaryotic genes have introns, which must be spliced out before transcribed RNA can be translated into protein. And yes, the genomes of eukaryotic cells swap genes (aka sexual recombination).
Eukaryotes are the result of an ancestral archaebacterium that swallowed up a eubacterium (see pic step #1). The “archaea” gave rise to the nuclear genome the eubacterium gave rise to mitochondria. If eukaryotes had a moto it would be “you don’t have to out-race the competition, when you can eat them”.
In the article, William Martin from the University of Dusseldorf and Eugene V. Koonin from the NIH propose that the nucleus evolved to physically segregate the production and processing of RNA from the act of translating the RNA into proteins.
So why was this necessary?
They contend that the mitochondrial ancestor must have carried parasitic genes that can copy and paste themselves within the mitochondrion genome. Similar genes, or disarticulate group II introns, have been found in some modern eubacteria. Fortunately these parasites when in their RNA form, can splice themselves out of any RNA transcripts. (RNA + catalytic power = Ribozymes!) OK lets get back to the story … after being swallowed, mitos replicated and some inevitably died (step#2). The genomes of the dead mitos were then absorbed into the what was the archeabacterial genome (step#3). But now comes the crunch … mito based parasitic DNA could spread into the archea based genome. Sometimes the junk would mutate and lose the ability to self-splice, however these could still be spliced out by other functional parasitic junk. Apparently there is data that functional group II introns can splice out other introns that have lost the ability to self-splice. These catalytic RNAs became essential and gave rise to the eukaryotic splicing machinery … aka the snRNAs. The last problem faced was that RNA splicing is much slower than RNA translation. To prevent translation from occurring on unspliced RNAs, the two machineries were segregated into separate compartments: RNA processing in the nucleus and RNA translation in the extra-nuclear, or cytoplasmic, space (step#4).
But there’s more!
In eukaryotes, sheets derived from the endoplasmic reticulum (ER) form the bilayered nuclear membrane. Entry and exit from the nucleus is controlled by nuclear pores. According to the authors (although I’ll have to check this out) the major components of the ER are derived from archeabacterial genes. This would indicate that the ER was already present in our archaea-anscestor before it swallowed mitos. If we look at nuclear pore components, or nuclear specific proteins, some are derived from archeabacteria genes and some from eubacteria … thus the nucleus was pieced together using genes from both the host and mito genome … (drum roll) … and thus the nucleus arose AFTER mitos arrived.
Great story! Let’s see if it holds up.
Ref: William Martin and Eugene V. Koonin, Introns and the origin of nucleus–cytosol compartmentalization. Nature (2006) 440:41-45