Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 16

The archaea (formerly called archaebacteria) are single-celled organisms that resemble bacteria. Bacteria and archaea were previously considered one group called the prokaryotes, which means “before the nucleus,” and are still sometimes referred to by this name. However, there are hazards with this designation, as it erroneously implies that bacteria and archaea are more closely related to each other than they are to eukaryotes (see below). The term “prokaryote” also tends to obscure the idea that present-day bacteria and archaea have had as much time to evolve as eukaryotes since they separated and so did not “come before.” Bacteria and archaea do lack a nucleus, the defined membrane structure found in eukaryotes that houses the vast majority of the genes of the organism. The presence or absence of a nuclear membrane greatly influences the mechanism used to manufacture proteins in the cell. The processes of messenger RNA (mRNA) synthesis and translation are coupled in bacteria and archaea, since no nuclear membrane separates the ribosomes (which synthesize proteins) from the DNA (see chapter 2). However, in most eukaryotes, mRNA made in the nucleus must be transported through the nuclear membrane before it can be translated into protein in the cytoplasm, where the ribosomes reside, and transcription and translation do not occur simultaneously.

Besides lacking a nucleus, bacterial and archaeal cells lack many other cellular constituents common to eukaryotes, including mitochondria and chloroplasts. They also lack such visible organelles as the Golgi apparatus and the endoplasmic reticulum. While bacteria can possess compartmentalized features for a wide variety of purposes such as the storage of carbon and nutrients and for specialized enzymatic processes, the absence of most organelles generally gives bacterial and archaeal cells a much simpler appearance under the microscope than eukaryotes.

Extremophiles (or “extreme-condition-loving” organisms), as their name implies, live under extreme conditions where other types of organisms cannot survive, such as at very high temperatures, in highly acidic environments, and at very high osmolality, such as in the Dead Sea. Most extremophiles are archaea. However, it is becoming clear that archaea also are important components of many less extreme environments; for example, archaea perform unique biochemical functions, such as making methane, and can be normal inhabitants of the human microbiome.

The archaea themselves are a very diverse group of organisms, and our understanding of the phylogeny of archaea is an area of intense research that is currently in flux. One exciting update to our understanding is that it is now clear that eukaryotes branch out of the archaea and, more specifically, out of the TACK superphylum (Thaumarchaeota, Aigarchaeota, Crenarchaeota, and Korarchaeota). Perhaps the most exciting area of research involves the discovery of uncultured archaeal lineages like the Lokiarchaeota that possess numerous molecular systems that were previously only associated with eukaryotes, providing a clear link bet ween archaea and eukaryotes (see Spang et al., Suggested Reading). Excitingly, an archaeon that appears to be on the border between prokaryotes and eukaryotes has now been isolated and is pictured at the start of this chapter (see Imachi et al., Suggested Reading). The relationship between the superphylum TACK archaea and two other large divisions with the archaea, the Euryarchaeota and the superphylum DPANN (Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota, and Nanohaloarchaeota), is currently under investigation. While the basic molecular processes of archaea are an active area of investigation, much less is known about the archaea than about the bacteria.