Cell Cycle And Polyploidy In Haloarchaea

Halobacterium salinarum has a very strict cell cycle control and stops cell division when replication is inhibited. Synchronized cultures were used to quantify cell cycle-specific changes of the transcriptome and the proteome. The number of cycling transcripts and proteins is much smaller than in other species. An explanation might be the surprising finding that Hbt. salinarum does not have an S-phase but replicates constitutively. It is the first species found to have replication uncoupled from other cell cycle processes like intracellular DNA transport and cell division. Hbt. salinarum, Haloferax volcanii and Haloferax mediterranei are all polyploid with 15 to 25 copies of their major chromosome. It is tempting to speculate that polyploidy might be typical for haloarchaea. The copy numbers of the minor chromosomes and plasmids differ from those of the major chromosome and from one another, thus the dosage of haloarchaeal genes depends on their localization on different replicons and can differ by as much as a factor of five. Nine different possible evolutionary advantages of polyploidy for haloarchaea are discussed, including a low mutation rate, high desiccation/X-ray irradiation resistance, survival over geological times or at extraterrestrial places, gene redundancy enabling the existence of heterozygous cells, relaxation of replication control, global gene dosage control, statistical instead of stochastical regulation of gene expression, and the usage of DNA as phosphate storage polymer. The majority of these evolutionary advantages require the presence of intermolecular gene conversion, which could indeed be shown to exist in haloarchaea.