Are there eukaryotic bacteria

A prokaryotic cell is a simple, single-celled unicellular organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes.

Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid Figure 1. Unlike Archaea and eukaryotes, bacteria have a cell wall made of peptidoglycan, comprised of sugars and amino acids, and many have a polysaccharide capsule Figure 1.

The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion, while most pili are used to exchange genetic material during a type of reproduction called conjugation. In nature, the relationship between form and function is apparent at all levels, including the level of the cell, and this will become clear as we explore eukaryotic cells.

It means that, in general, one can deduce the function of a structure by looking at its form, because the two are matched. In prokaryotes, DNA is bundled together in the nucleoid region, but it is not stored within a membrane-bound nucleus. The nucleus is only one of many membrane-bound organelles in eukaryotes. Prokaryotes, on the other hand, have no membrane-bound organelles.

Another important difference is the DNA structure. Eukaryote DNA consists of multiple molecules of double-stranded linear DNA, while that of prokaryotes is double-stranded and circular. A comparison showing the shared and unique features of prokaryotes and eukaryotes. All cells, whether prokaryotic or eukaryotic, share these four features:.

Plasma membrane. In prokaryotic cells, transcription and translation are coupled, meaning translation begins during mRNA synthesis. In eukaryotic cells, transcription and translation are not coupled.

But this version of events ignores the fact that, for the past few decades, researchers have been quietly uncovering many complex structures within prokaryotes, including membrane-bound organelles. In contrast to eukaryotes, which all have a suite of organelles in common, different groups of prokaryotes showcase their own specialized compartments. One kind of bacterial organelle, discovered in , is essentially a little magnet wrapped in a lipid package; another hosts a series of reactions crucial for energy metabolism; still others serve as small storage units for nutrients.

And that list is only growing as scientists discover more and more compartments within supposedly simple bacterial cells. The very existence of organelles in these bacteria, coupled with intriguing parallels to the more familiar ones that characterize eukaryotes, has prompted scientists to revise how they think about the evolution of cellular complexity — all while offering new ways to probe the basic principles that underlie it.

Yet, while eukaryotic organelles have been studied in great detail for many decades, it has only recently become possible to do so in prokaryotes. That made it extremely difficult to isolate and analyze bacterial compartments to get a sense of what they were — and what they were doing. Archaea, which were only recognized as a distinct prokaryotic kingdom in the s, have received even less scrutiny than bacteria. Better imaging techniques eventually started to make such research easier.

An electron micrograph of the bacterium Magnetospirillum magneticum top reveals the chain of magnetosomes it uses to navigate.

A lipid membrane surrounds each magnetic particle closeups at bottom. These structures, which are among the best-studied prokaryotic organelles, allow the bacterium to navigate through its aquatic environment.

Among the best studied of the bacterial organelles are the magnetosomes, round structures that build magnetic particles within their lipid bilayer membranes. Komeili and his colleagues have been identifying the genes and proteins involved in how magnetosomes are built, maintained and later divided among cellular offspring.

But magnetosomes are not alone. Scientists have stumbled on a plethora of other bizarre bacterial compartments , often while searching for something else. Although many of these might not be considered organelles by the strictest definitions — organelles have to be lipid-bound structures completely separated from the cell membrane — some of them do fit the bill. In fact, it is so beautiful that in the United States, the anti-evolutionary creationists seized upon it as being something so fantastic that it could not possibly have evolved [ 86 ].

Happily there is actually very nice structural evidence that evolution of the flagellar rotor has indeed occurred [ 87 ]. There are other several kinds of biological motors that can convert chemical energy into mechanical energy, and it is convenient to classify all of the biological motors we know about into five classes, which are not really mutually exclusive. The rotary motors such as the flagellar rotor would be one. Linear stepper motors, like kinesin, myosin and dynein, would be another [ 88 ].

Assemby and disassembly motors - using the forces that you get from polymerization of and depolymerization of microtubules or actin - make up another class [ 70 ]. Or there can be pre-stressed springs that are built in such a way that they store mechanical energy that can be released all at once, as, for example, in the acrosomal reaction in the horseshoe crab sperm [ 89 ].

And then there are also extrusion nozzles, where a cell will squirt out very hygroscopic polysaccharide that can allow it to jet along.

Myxococcus xanthus does that [ 90 ]. Bacteria have some examples of all of those classes of biological motors. There is nothing known that does linear stepping on FtsZ. Why should it be so difficult? Looking just at the linear stepper motors for microtubules and actin, there are three major classes [ 88 ]. There are the myosins for actin, and the kinesins and dynein for microtubules.

It has been shown structurally - and this was a real surprise for me and I think for most people - that kinesin and myosin have very similar central folds around the region where they couple nucleotide hydrolysis to piston-like motion, and are almost certainly derived from a common ancestor [ 91 , 92 ]. Dynein is definitely the odd man out.

It is a very different kind of motor, related to a completely different class of ATPases. It has been speculated that there was some kind of motor precursor that was the common ancestor of myosin and kinesin [ 93 ]. But the heart of both of those motors is the nucleotide switch that converts hydrolysis into a large-scale protein conformational change resulting in stepping movement.

Other aspects of motor function, such as the binding to the filament, are quite different among different motors, and if you look even just within the families - the myosin family, the kinesin family - the way they couple that nucleotide switch to motion is actually very wildly, dramatically different among different individuals [ 94 ].

For example, most myosins walk toward the barbed end of the polarized actin filament, but one particular subfamily, myosin VI, walks in the opposite direction toward the pointed end [ 95 , 96 ]. Focusing on the nucleotide switch at the heart of the motor, these cytoskeletal molecular motors are members of what is called the P-loop NTPase family. There has been a heroic attempt made by Eugene Koonin and colleagues to classify all of these many very divergent proteins into a reasonable phylogenetic tree based on sequence and structural similarities [ 97 ].

Given that this is such a diverse protein family spanning essentially the whole history of cellular evolution, there is some uncertainty here, but one thing about their reconstructed phylogeny really leapt out at me. According to their analysis, there is a entire branch of the P-loop NTPases that is found only in eukaryotes, and not in bacteria or archaea.

This branch includes not only myosin and kinesin, but also many other critical proteins that we associate with eukaryotic cellular complexity. These include the Rho GTPase superfamily, which act as master regulators for actin cytoskeletal assembly [ 98 ], the Rab GTPases that govern many aspects of membraneous organelle identity [ 99 ], the Arf GTPases that are also associated with membrane traffic [ ], the Ran GTPase that governs the directionality of nuclear import and export [ ], and the heterotrimeric G proteins that influence so many aspects of eukaryotic cell-to-cell signaling [ ].

So, wow. This looks very much like the list of eukaryotic-specific cellular features that we started off with. It seems historically as if a branch of the P-loop NTPase family might have arisen in eukaryotes at some point when they had presumably already been evolutionarily separated from the bacteria and the archaea, and this novel protein family gave rise not just to the myosins and kinesins, but also to many of the regulatory and signaling proteins that we most closely associate with the eukaryotic way of life.

Bacteria, of course, have very good signalling proteins, such as the large family of two-component signal transduction systems involving histidine kinases and response regulators [ ]. Who knows why that happened - maybe it was just good luck, maybe the innovation that led to those branches of the P-loop NTPase superfamily is something that happened in eukaryotes so that they were able to seize advantage of it and then combine it with their other properties and develop the ability to make these very large and elaborate, well organized and polarized cytoskeletal structures that would enable them to do things like build a mitotic spindle.

Bacteria already had a perfectly good strategy going without these kinds of systems. Arguably in many ways the prokaryotic side of the tree, the bacteria and archaea, are much more diverse and more successful than eukaryotes - certainly there are many more of them than there are of us.

They are particularly good at diversifying their metabolisms. All of the really exciting inventions in biological chemistry, I would say, have been generated in the prokaryotic branches of the tree. Photosynthesis, for example, is simply an awesome idea, and it was cyanobacteria that came up with that. Eukaryotes never could come up with that whole crazy business about using a cubic manganese cluster to strip the electrons off of water [ ]. The best that eukaryotes could do was to tame the cyanobacteria and get them to come and live inside and become chloroplasts.

I think the bacterial strategy is terrific, it is just different from our eukaryotic strategy. Our strategy has much more to do with morphological diversification, including getting very large both as cells and as organisms, and developing hunting strategies of various different kinds.

I think this is probably both a consequence and a cause in a feedback loop mechanism of the diversification of cytoplasmic cytoskeletal structures that then gave rise to larger-scale morphological diversity in eukaryotes. This fourth part of my argument is now much more speculative than even the most speculative parts of what I have said before. Let us stipulate that it is observable that all cells are organized in some way. What is their central organizing principle? Where is the information that is used by various different components of the cell to know where they are in relationship to everyone else?

In most bacteria there are only one or a few chromosomes. They tend to be oriented in a very reproducible way as you go from one individual to the next [ , ] and because of the coupled transcription and translation, the physical site where you have a bit of DNA is also connected to the physical site where you make the RNA and the physical site where you make the protein from that bit of information [ ].

If it is important to a bacterial cell to be able to target something to a specific location, it already has all the information it could ever hope for about which location in the cytoplasm is which because it has a well-defined, oriented chromosome present there. It is a very difficult chicken-and-egg problem as to what came first. Was it the wrapping of the nucleus that caused the actin and tubulin cytoskeletons to expand their capacities, or was it the explosion of the capacity of the cytoskeleton that wrapped up the nucleus in membrane?

I like to imagine that at some point the nucleus got sequestered away somehow by some sort of prototypical membrane, maybe like what we see now in Gemmata , and then the poor little cytoskeletal elements were left out there in the cytoplasm on their own.

They had no way of knowing where they were or of measuring space or position. So they had to figure out how to do it by themselves, without the chromosome there to help. Our eukaryotic cytoskeletons figured out how to do this by setting up large-scale arrays that can be oriented by virtue of having nucleators and molecular motor proteins to make those type B structures that are so useful for spatial organization over vast distances of many tens of micrometers.

I think that this is a very elegant solution. The other benefit that the eukaryotes may have gotten from this strategic decision is extra morphological evolvability. In one of your other interviews, Marc Kirschner made some very interesting points about how certain kinds of preexisting conditions may make it relatively easy for some animal lineages to generate highly variable morphology [ ].

I think the eukaryotic cytoskeleton may well be an example of this at the cellular level, an idea that Marc also certainly shares [ ]. Once the lonely but inventive eukaryotic cytoskeletal proteins committed to the strategy of using a very small number of filament types to perform a large number of different functions, the addition of a new kind of organizational function to the underlying cytoskeletal framework may have been as simple as coming up with a few new modulators of cytoskeletal filament dynamics, or another kind of slightly modified motor protein.

This diversification may have happened very quickly on an evolutionary scale. Sequence analysis of the myosin and kinesin motor families seems to suggest that the most recent common ancestor for all the currently living eukaryotes already had several different kinds of each motor [ , ]. Indeed this most recent common ancestor may even have been capable of both amoeboid crawling motion and flagellar swimming [ ].

It may be that the bacteria just never had to face this particular problem because, again, almost universally they have kept their chromosome right there in the cytoplasmic compartment where they could use it for spatial information. So typically, when a particular bacterium needs to make a filamentous structure for a novel purpose, such as orienting the magnetosomes in Magnetospirillum [ 5 ], it duplicates the gene for a cytoskeletal filament and adapts it for that one new purpose.

This works fine for the purpose at hand, but forgoes the opportunity for flexibility and truly large-scale cellular organization that are intrinsic features of both the eukaryotic actin and microtubule cytoskeletons.

Knowing eukaryotes, I would guess that the ones that figured out how to do phagocytosis first just ate everybody else. At some point initially, the earliest eukaryote must have looked much like its contemporary bacterial and archaeal counterparts, but it had secrets inside it that enabled it to become different. I think the fact that you see that both the diversification of the important NTPase families and the elaboration of cytoskeletal functions seem to be universal among eukaryotes means that probably those things happened relatively quickly.

So I suspect the original eukaryote was small. I suspect it was pretty simple-looking compared with Stentor or one of the really fabulous single-celled eukaryotes. PLoS Biol. EMBO J. Raven PH: A multiple origin for plastids and mitochondria. Google Scholar.

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