Procaryotes

Procaryotes are organisms without a nucleus. Bacteria and blue-green algae (Cyanophyta, Cyanobacteria) belong to them. Procaryotes were once believed to belong to a uniform organic kingdom, but by now irrefutable evidence that they belong to at least two different kingdoms whose cells differ considerably both in structure and metabolism exist. These two kingdoms are:

archaebacteria

eubacteria, including blue-green algae.

These two kingdoms have, nevertheless, some things in common, especially the structures required for the maintenance, expression, and the transmission of genetic information ( see hypercycles). The ribosomes of both kingdoms contain besides a number of proteins ribosomal RNA (rRNA) and tRNA. The ribosomal structure and both types of RNA are among the most conservative supra-molecular complexes found in all cells. The selection pressure working in favor of the maintenance of reliable structures is very high. Yet, even here, modifications occurred in the course of time. Due to their omnipresence, RNA and ribosomes are especially well-suited as markers in the study of the relations between the different biological kingdoms. Lab techniques make a confinement to the analysis of RNA sensible, and it is not necessary to learn the base sequences in order to understand the degree of relationship and phylogenetic connections. Methods of approximation give just as conclusive results.

Sequence analysis, that can due to the relatively high effort be performed with just a small number of species, may supply additional information. If the sequence of a polynucleotide chain is known, the secondary structure can be determined by optimization. The secondary structure proved itself to be an even more conservative marker than the presence of certain bases in a given position. Secondary structures remain, even if the nucleotide sequences changed considerably in the course of time, so that homologies can not be determined by comparison of two sequences alone. For the formation of base pairs, an A has not necessarily to have a U opposite itself, a change to a G and a C changes the situation but little. Within a sequence of base pairs, even ‘wrong’ pairs may occur, because C and A or other base combinations form hydrogen bonds, too, that help stabilizing the secondary structure. This argument seems to be inconsistent with what was deduced during the discussion of the evolution of genetic information. The contradiction is nevertheless just a seeming one, as rRNA is a structural component. The base pair sequence contains no genetic information.

A comparison of the rRNA from the chloroplasts of green plants with that of blue-green algae shows, how conservative a secondary structure can be: their structures are highly homologous, data that belongs, as you will see later, to the safest pillars of the endosymbiontic theory.

The analysis of rRNA, on the other hand, shows that enormous differences between archaebacteria and eubacteria exist, and that the variability of the rRNA within the archaebaceria is far larger than that within the eubacteria. The combination of all rRNA data shows, that very early during the diversification of the organisms, three separate evolutionary lines evolved that led to the archaebacteria, the eubacteria, and the eucaryotes.

It is assumed, that hypothetical primitive cells, so-called protocells were the common ancestors of all three lines. Procaryotic cells are with few exceptions like Mycoplasma or Thermoplasma, surrounded by a cell wall. The chemical composition and the structure of the cell wall is subject to a large variability within the archaebacteria group. The cell walls of eucaryotes, in contrast, are mostly uniform. They consist of one or several layers of murein as is easily shown by Gram’s staining method. Due to this reaction, it is distinguished between gram-positive and gram-negative bacteria. This classification turned out to be a reliable taxonomic feature.

The genetic information of prokaryotes is usually stored in a circular molecule of DNA. In archaebacteria, gene-splicing occurs, which means that the information required for the production of a polypeptide chain is distributed onto several pieces, so-called exons that are separated from each other by non-coding sequences called introns. This type of organization is not known to occur in eubacteria, too. In some well-studied species, like Escherichia coli, gene splicing does not take place meaning that the genome of these species has been optimized during evolution and that all useless genetic sequences have been lost. The complete nucleotide sequence of strain K12 is known. It contains 4,639,221 base pairs corresponding to 4,288 genes. 38% of these genes have not yet been assigned an essential function within the cell. This again is a hint, that organisms have a far larger supply of genetic information than required for their survival. It seems that the genome is organized in a three-dimensional fashion and that it is well-suited for horizontal gene transfer, since it contains insertion elements and sequences with unusual compositions (BLATTNER, F.R. et al. (16 co-authors), 1997).

Besides from the single molecule of DNA, a bacterium contains numerous additional small molecules of DNA, the plasmids. They, too, carry genetic information, especially such information that is not always required but may prove life-saving under certain conditions, like, for example, resistances against antibiotics.

Some plasmids are able to integrated into the bacterial chromosome. Also, part of the bacterial chromosome may be cut out and incorporated into a plasmid. Plasmids provide a possibility to transfer genetic information from one cell to another, different species being no insuperable barrier. This process is called horizontal gene transfer. Usually, the consistency of the genome of the respective species remains intact as the foreign DNA entering the cell is recognized and degraded. Bacteria have an extensive array of restriction endonucleases specific for this purpose at their disposal. The bacterial genome is mostly haploid, but the information stored in the plasmids has to be regarded as polyploid, since the cells contain usually several copies of the same type of plasmid.

Beside the exchange of genetic information via plasmid, the cell is able to gain valuable information by taking up DNA. These mechanisms are generally referred to as parasexuality. In contrast to the sexuality of eucaryotic cells, where the genetic information of both partners is equal and where both genomes contribute equally to the species’ progeny, in parasexuality, the information of one partner is enhanced at the expense of the other’s.

All prokaryotes have 70 S ribosomes. The protein biosynthesis occurring at these ribosomes is inhibited by chloramphenicol.

Bacterial cells are usually very small. It has been calculated that their volume is not large enough to harbor all gene products encoded by the bacterial cell.

Due to a lack of morphological variation, bacteria are classified according to their metabolic performance. The already discussed performances of chemolitoautotropic, heterotrophic, and photoautotrophic bacteria give a good impression of the variety of bacterial metabolisms. Moreover, each cell is able to regulate its metabolic activity according to need. Certain enzymes, especially those participating in the carbohydrate metabolism are only produced when needed, i.e. at sufficient levels of the respective substrate in the medium. Otherwise, the synthesis of these enzymes is inhibited. This regulation makes for lesser expenses of energy, space, and material. The mechanisms of regulation are extraordinarily variable. They involve the substrate itself, a substrate-specific regulator protein, and a section of DNA that is recognized by the regulator protein. The regulator-DNA complex inhibits the transcription of the subsequent gene or genes and thus also the production of the enzyme (or enzymes) necessary for the catalysis of the respective substrate. These mechanisms show that on a procaryotic level, processes exist that have to be regarded as differentiations.

Many species form permanent cells under unfavorable conditions that are not active metabolically. When the extern circumstances are favorable again, the permanent cells may regenerate into fully functional cells.

Bacteria are usually unicellular. The cells of many species may nevertheless combine in regularly structured aggregates also called colonies. Among these aggregates are multicellular filaments and polar trichomes that are screw-like or bundled. Spherical bacteria (cocci) are organized in regular patterns, usually in groups of four. Such colonies are often surrounded by a voluminous jelly coat. Differently structured cells occurring in the same association as is the case with myxobacteria and actinomycetes hint at a division of function. Some aggregations look much like the mycelia of fungi.