Effect of growth rate on the macromolecular composition of Prototheca zopfii, a colorless alga which divides by multiple fission

Membranes, energetics, and evolution across the prokaryote-eukaryote divide

The evolution of the eukaryotic cell marked a profound moment in Earth’s history, with most of the visible biota coming to rely on intracellular membrane-bound organelles. It has been suggested that this evolutionary transition was critically dependent on the movement of ATP synthesis from the cell surface to mitochondrial membranes and the resultant boost to the energetic capacity of eukaryotic cells. However, contrary to this hypothesis, numerous lines of evidence suggest that eukaryotes are no more bioenergetically efficient than prokaryotes. Thus, although the origin of the mitochondrion was a key event in evolutionary history, there is no reason to think membrane bioenergetics played a direct, causal role in the transition from prokaryotes to eukaryotes and the subsequent explosive diversification of cellular and organismal complexity.

Over time, life on Earth has evolved into three large groups: archaea, bacteria, and eukaryotes. The most familiar forms of life – such as fungi, plants and animals – all belong to the eukaryotes. Bacteria and archaea are simpler, single-celled organisms and are collectively referred to as prokaryotes.

The hallmark feature that distinguishes eukaryotes from prokaryotes is that eukaryotic cells contain compartments called organelles that are surrounded by membranes. Each organelle supports different activities in the cell. Mitochondria, for example, are organelles that provide eukaryotes with most of their energy by producing energy-rich molecules called ATP. Prokaryotes lack mitochondria and instead produce their ATP on their cell surface membrane.

Some researchers have suggested that mitochondria might actually be one of the reasons that eukaryotic cells are typically larger than prokaryotes and more varied in their shape and structure. The thinking is that producing ATP on dedicated membranes inside the cell, rather than on the cell surface, boosted the amount of energy available to eukaryotic cells and allowed them to diversify more. However, other researchers are not convinced by this view. Moreover, some recent evidence suggested that eukaryotes are no more efficient in producing energy than prokaryotes.

Lynch and Marinov have now used computational and comparative analysis to compare the energy efficiency of different organisms including prokaryotes and eukaryotes grown under defined conditions. To do the comparison, the results were scaled based on cell volume and the total surface area deployed in energy production.

From their findings, Lynch and Marinov concluded that mitochondria did not enhance how much energy eukaryotes could produce per unit of cell volume in any substantial way. Although the origin of mitochondria was certainly a key event in evolutionary history, it is unlikely to have been responsible for the diversity and complexity of today’s life forms.

RNA:protein ratio of the unicellular organism as a characteristic of phosphorous and nitrogen stoichiometry and of the cellular requirement of ribosomes for protein synthesis

BACKGROUND

Mean phosphorous:nitrogen (P:N) ratios and relationships of P:N ratios with the growth rate of organisms indicate a surprising similarity among and within microbial species, plants, and insect herbivores. To reveal the cellular mechanisms underling this similarity, the macromolecular composition of seven microorganisms and the effect of specific growth rate (SGR) on RNA:protein ratio, the number of ribosomes, and peptide elongation rate (PER) were analyzed under different conditions of exponential growth.

RESULTS

It was found that P:N ratios calculated from RNA and protein contents in these particular organisms were in the same range as the mean ratios reported for diverse organisms and had similar positive relationships with growth rate, consistent with the growth-rate hypothesis. The efficiency of protein synthesis in microorganisms is estimated as the number of active ribosomes required for the incorporation of one amino acid into the synthesized protein. This parameter is calculated as the SGR:PER ratio. Experimental and theoretical evidence indicated that the requirement of ribosomes for protein synthesis is proportional to the RNA:protein ratio. The constant of proportionality had the same values for all organisms, and was derived mechanistically from the characteristics of the protein-synthesis machinery of the cell (the number of nucleotides per ribosome, the average masses of nucleotides and amino acids, the fraction of ribosomal RNA in the total RNA, and the fraction of active ribosomes). Impairment of the growth conditions decreased the RNA:protein ratio and increased the overall efficiency of protein synthesis in the microorganisms.

CONCLUSION

Our results suggest that the decrease in RNA:protein and estimated P:N ratios with decrease in the growth rate of the microorganism is a consequence of an increased overall efficiency of protein synthesis in the cell resulting from activation of the general stress response and increased transcription of cellular maintenance genes at the expense of growth related genes. The strong link between P:N stoichiometry, RNA:protein ratio, ribosomal requirement for protein synthesis, and growth rate of microorganisms indicated by the study could be used to characterize the N and P economy of complex ecosystems such as soils and the oceans.

Cell cycle control by timer and sizer in Chlamydomonas

Conservation of cell cycle control mechanisms is indicated by the presence of functionally homologous division control genes in unrelated yeasts and by the nonspecific action of oncogenes, but it remains uncertain what property of a growing cell results in the initiation of events leading to division. Response to a critical size is indicated by the longer growth period of smaller cells prior to division, which is consistent with deferment of division events until a minimum size is attained; however, in the same cell types faster growing cells are larger and this is more easily explained if division follows a timed period during which faster growing cells grow more, as is postulated for mammalian cells. Therefore, either time- or size-dependent controls might be the sole significant mechanism; we report here, however, that both controls do function in Chlamydomonas since cycle duration is under timer control and cell size determines the number of division rounds committed at the end of each cycle, and hence whether 2, 4, 8 or 16 daughter cells are formed.

Coordination of macromolecular synthesis in the slime mould Physarum polycephalum

Microplasmodia of P. polycephalum were grown either in batch culture, in both complex and defined media to give a 3-4 fold variation in growth rate, or in a chemostate. The protein/DNA ratio of batch cultures was almost invariant, whilst the RNA/DNA ratio increased as a non-linear function of growth rate. The amount of ribosomal RNA, expressed as a fraction of total RNA, showed little variation and this was also true for the proportion of ribosomes found in polyribosomes. Calculation of the rate of protein synthesis per ribosome shows that this parameter increases by approximately 50% over the range of growth rates studied, although it should be emphasized that the effect of protein turnover has not yet been taken into account. Enrichment of batch cultures growing in a defined medium produced an increase in the rate of RNA synthesis. Data obtained with chemostat cultures differed in several respects from those described above for batch cultures, especially at low growth rates, and are discussed in relation to the early stages of differentiation of microplasmodia to spherules.

Effect of growth rate on the amounts of ribosomal and transfer ribonucleic acids in yeast

The steady-state growth rate of Saccharomyces cerevisiae was varied by growing the cells in different media. The total amount of ribonucleic acid (RNA) per cell was found to decrease as a nonlinear function of decreasing growh rate. The RNA from cells growing in different media was analyzed by polyacrylamide gel electrophoresis. Although the amounts of both ribosomal RNA and transfer RNA decreased with decreasing growth rate, the ratio of ribosomal to transfer RNA was not constant. As the growth rate was reduced the ribosomal RNA fraction decreased slightly, whereas the transfer RNA fraction increased slightly. Thus the levels of ribosomal and transfer RNA were regulated to similar yet different extents. The levels of the different ribosomal RNA species were more closely coordinated. At all growth rates the ribosomal RNAs (including 5S RNA) were present in equimolar amounts. The rate of protein synthesis in yeast cells also decreased with decreasing growth rate. The low rates of protein synthesis did not appear to be due to limiting numbers of ribosomes or transfer RNA molecules.

Ecological application of antibiotics as respiratory inhibitors of bacterial populations

Two terregenous and four marine bacterial isolates were treated with six antibiotics and antibiotic combinations. Comparisons made between responses of cells in early and late logarithmic and stationary growth phases indicated variable sensitivity to the agents. Bacteria in stationary and late log-phase cultures exhibited the greatest resistance, whereas the early log-phase cells exhibited greatest antibiotic susceptibility. We conclude that the tested antibiotics cannot be used for ecological purposes to delineate bacterial respiration in mixed microbial communities.