Nucleoid structure and distribution in thermophilic Archaea

Abstract

Nucleoid structure and distribution in thermophilic organisms from the Archaea domain were studied. Combined phase-contrast and fluorescence microscopy of DAPI (4,6-diamidino-2-phenylindole)-stained Sulfolobus acidocaldarius and Sulfolobus solfataricus cells revealed that the nucleoids were highly structured. Different nucleoid distribution within the cells, representing different partition stages, was observed. The conformation of the nucleoids differed between exponentially growing and stationary-phase cells. Also, the stationary-phase cells contained two chromosomes, and the nucleoids occupied a larger part of the interior of the cells than in the exponentially growing cells. The part of the cell cycle during which fully separated nucleoids could be detected was short. Since the postreplication period is long in these organisms, there was a considerable time interval between termination of chromosome replication and completion of nucleoid separation, similar to the G 2 phase in eukaryotic cells. The length of the visible cell constriction period was found to be in the same range as that of eubacteria. Finally, cell-cell connections were observed under certain conditions. Possible eubacterial, eukaryotic, and unique features of nucleoid processing and cell division in thermophilic archaea are discussed.Chromosome replication, segregation of the replicated genomes, and cell division are key processes in the cell cycle of any organism. The mechanistic and regulatory features of these processes have been extensively studied in eukaryotes and eubacteria. In contrast, little is known about the cell cycle in organisms from the third major evolutionary lineage of life on earth, the archaea (24).When the complete genome sequence of the archaeon Methanococcus jannaschii become available (7), several genes that may be involved in replication, partition, and cell division (called cell cycle genes in this work) in archaeal cells were found. Genes similar to cell cycle genes of organisms from both the Bacteria (e.g., ftsZ, minD, and soj) and the Eukarya (e.g., CBF5, cdc21, CDC54, and X.MCM4) domains were identified, which indicates that insights into the cell cycle of archaea may also help to increase the understanding of the cell cycle in organisms from the other two domains (see discussion in reference 3).Insight into the cell cycle features of archaea can be obtained not only from the M. jannaschii gene repertoire. Cell morphology has been used to draw conclusions about the mode of growth and division of different archaea (reference 13 and references therein). The genus studied in this report, Sulfolobus, was first described morphologically by Brock et al. (6).By staining the chromosome DNA (the nucleoids) in situ, nucleoid structure and distribution may be studied by light microscopy. This information can be used to gain insight into the intracellular organization of the chromosome DNA, the morphological features of the nucleoid partition process, and the temporal stage in the cell cycle at which different nucleoid pro...

Genome-wide transcription map of an archaeal cell cycle

“…Furthermore one, or both, of the two Sac7 DNA-binding proteins displayed strong cyclic induction around the S/G 2 transition, indicating involvement in cell-cycle-specific chromatin rearrangement. The chromosome DNA in Sulfolobus cells seems to occupy a smaller relative intracellular space during genome segregation (10,11). This finding indicates that the chromatin condensation state changes during the cell cycle, and it is possible that altered acetylation status, or differences in the abundance of DNA-binding proteins, could influence higherorder DNA structure.…”

“…The cell cycle displays several eukaryotic-like features, including synchronous replication initiation from multiple origins (12,13), a ten-fold lower replication rate than in bacteria (12), and a distinct G 2 phase between termination of chromosome replication and initiation of genome segregation (10,11). It is, consequently, possible that the mitotic and division machineries also display similarities to eukaryotic counterparts (6).…”

“…Here, we report a global analysis of relative RNA abundance at different cell-cycle stages in S. acidocaldarius, using synchronized populations and microarraybased transcript profiling. resuspension, respectively, because each process occupies Յ5% of the cell cycle (10,11). After division, the cells proceeded through a short G 1 period followed by rapid entry into S phase (9).…”

“…The cell cycle of organisms belonging to the archaeal genus Sulfolobus is characterized by a short G 1 phase, an S phase of about a third of a cell generation, and an extensive G 2 phase lasting more than half the generation time, followed by rapid genome segregation and cell division (9)(10)(11). The cell cycle displays several eukaryotic-like features, including synchronous replication initiation from multiple origins (12,13), a ten-fold lower replication rate than in bacteria (12), and a distinct G 2 phase between termination of chromosome replication and initiation of genome segregation (10,11).…”

Three replication origins in Sulfolobus species: Synchronous initiation of chromosome replication and asynchronous termination

“…The organisms are hyperthermophilic acidophiles that display optimal growth at Ϸ80°C and pH 3 and were originally isolated from geothermal hot springs (3,4). We have initiated studies of the cell cycle in the two species, with the main focus on chromosome replication, genome segregation, and cell division (5)(6)(7)(8)(9).…”