Gábor Vida

A NEW ASPECT TO THE ORIGIN AND EVOLUTION OF EUKARYOTES

Abstract. One of the most important omissions in recent evolutionary theory concerns how eukaryotes could emerge and evolve. According to the currently accepted views, the first eukaryotic cell possessed a nucleus, an endomembrane system, and a cytoskeleton but had an inefficient prokaryotic-like metabolism. In contrast, one of the most ancient eukaryotes, the metamonada Giardia lamblia, was found to have formerly possessed mitochondria. In sharp contrast with the traditional views, this paper suggests, based on the energetic aspect of genome organization, that the emergence of eukaryotes was promoted by the establishment of an efficient energy-converting organelle, such as the mitochondrion. Mitochondria were acquired by the endosymbiosis of ancient α-purple photosynthetic Gram-negative eubacteria that reorganized the prokaryotic metabolism of the archaebacterial-like ancestral host cells. The presence of an ATP pool in the cytoplasm provided by this cell organelle allowed a major increase in genome size. This evolutionary change, the remarkable increase both in genome size and complexity, explains the origin of the eukaryotic cell itself.The loss of cell wall and the appearance of multicellularity can also be explained by the acquisition of mitochondria. All bacteria use chemiosmotic mechanisms to harness energy; therefore the periplasm bounded by the cell wall is an essential part of prokaryotic cells. Following the establishment of mitochondria, the original plasma membrane-bound metabolism of prokaryotes, as well as the funcion of the periplasm providing a compartment for the formation of different ion gradients, has been transferred into the inner mitochondrial membrane and intermembrane space. After the loss of the essential function of periplasm, the bacterial cell wall could also be lost, which enabled the naked cells to establish direct connections among themselves. The relatively late emergence of mitochondria may be the reason why multicellularity evolved so slowly.

The origin of eukaryotes: the difference between prokaryotic and eukaryotic cells.

Eukaryotes have long been thought to have arisen by evolving a nucleus, endomembrane, and cytoskeleton. In contrast, it was recently proposed that the first complex cells, which were actually proto–eukaryotes, arose simultaneously with the acquisition of mitochondria. This so–called symbiotic association hypothesis states that eukaryotes emerged when some ancient anaerobic archaebacteria (hosts) engulfed respiring &agr;proteobacteria (symbionts), which evolved into the first energy–producing organelles. Therefore, the intracellular compartmentalization of the energy-converting metabolism that was bound originally to the plasma membrane appears to be the key innovation towards eukaryotic genome and cellular organization. The novel energy metabolism made it possible for the nucleotide synthetic apparatus of cells to be no longer limited by subsaturation with substrates and catalytic components. As a consequence, a considerable increase has occurred in the size and complexity of eukaryotic genomes, providing the genetic basis for most of the further evolutionary changes in cellular complexity. On the other hand, the active uptake of exogenous DNA, which is general in bacteria, was no longer essential in the genome organization of eukaryotes. The mitochondrion–driven scenario for the first eukaryotes explains the chimera–like composition of eukaryotic genomes as well as the metabolic and cellular organization of eukaryotes.

Speciation in Chlamydia: Genomewide Phylogenetic Analyses Identified a Reliable Set of Acquired Genes

Horizontal gene transfer (HGT), a process through which genomes acquire sequences from distantly related organisms, is believed to be a major source of genetic diversity in bacteria. A central question concerning the impact of HGT on bacterial genome evolution is the proportion of horizontally transferred sequences within genomes. This issue, however, remains unresolved because the various methods developed to detect potential HGT events identify different sets of genes. The present-day consensus is that phylogenetic analysis of individual genes is still the most objective and accurate approach for determining the occurrence and directionality of HGT. Here we present a genome-scale phylogenetic analysis of protein-encoding genes from five closely related Chlamydia, identifying a reliable set of sequences that have arisen via HGT since the divergence of the Chlamydia lineage. According to our knowledge, this is the first systematic phylogenetic inference-based attempt to establish a reliable set of acquired genes in a bacterial genome. Although Chlamydia are obligate intracellular parasites of higher eukaryotes, and thus suspected to be isolated from HGT more than the free-living species, our results show that their diversification has involved the introduction of foreign sequences into their genome. Furthermore, we also identified a complete set of genes that have undergone deletion, duplication, or rearrangement during this evolutionary period leading to the radiation of Chlamydia species. Our analysis may provide a deeper insight into how these medically important pathogens emerged and evolved from a common ancestor.

Urethane and hydroxyurethane induce sister-chromatid exchanges in cultured human lymphocytes.

Both urethane and hydroxyurethane induced sister-chromatid exchanges (SCE) in cultured human lymphocytes. Aroclor-induced rat-liver microsome fraction deactivated rather than activated these two agents in the lymphocyte system.

Role of metabolic activation in the sister chromatid exchange-inducing activity of ethyl carbamate (Urethane) and vinyl carbamate

Ethyl carbamate (EC, urethane) at 10(-2) M concentration induced more sister chromatid exchanges (SCEs) in cultured human peripheral blood lymphocytes in the absence of S9 mix than did 10(-2) M vinyl carbamate (VC), a possible proximate carcinogenic metabolite (Dahl et al., 1978) of EC. VC itself doubled SCE frequency over the control. In the presence of native S9 mix from Aroclor-induced rat liver, the SCE-inducing activity of VC was highly increased whereas that of EC was suppressed. This opposite effect of S9 mix on VC and EC seems to be due to two different factors. Activation of VC by the S9 fraction seems to be due to the presence of mixed-function oxidases in the S9 mix, because neither the native S9 fraction in the absence of co-factors nor the heat-inactivated S9 fraction in the incubation mixture led to the activation of VC. Deactivation of EC by S9 mix, on the other hand, seems to involve the presence of excess protein and/or substances of low molecular weight in the incubation mixture, because this deactivating effect did not change considerably when the S9 fraction was supplied in the absence of co-factors or when it originated from non-induced rat liver. Heat denaturation of the S9 fraction led to an increased deactivating effect on the SCE-inducing ability of EC. This result is in line with the assumption that reactive -SH groups in the S9 protein are at least partly responsible for the deactivation of EC by S9. Heat denaturation of the S9 fraction led to an about 1.5-fold increase in reactive -SH groups.