Social bacteria and asocial eukaryotes

The end of 2007 brought us draft genome sequences of two eukaryotic microorganisms, Babesia bovis and Malassezia globosa, as well as complete genomes of the ammonia-oxidizing archaeal chemoautotroph Nitrosopumilus maritimus, and several environmental bacteria (Table 1). In terms of genome size, this list covers both sides of the spectrum: the 245 kb genome of an obligate insect symbiont Sulcia muelleri is the second smallest microbial genome sequenced so far, whereas the 13 033 kb chromosome of the social myxobacterium Sorangium cellulosum breaks the record as the largest bacterial genome – and the largest DNA molecule in the prokaryotic world. Babesia bovis is an apicomplexan parasite of cattle that is closely related to two other animal pathogens, Theileria parva and Theileria annulata, whose genomes have been sequenced in 2005. It is also related to such human pathogens as malaria-causing Plasmodium spp. and Toxoplasma gondii, the causative organism of human toxoplasmosis. Like Plasmodium spp., B. bovis infects and eventually lyses erythrocytes, causing anaemia. Studies of B. bovis infection, besides helping protect the livestock, could lead to a better understanding of malaria. The description of B. bovis genome sequence (Brayton et al., 2007) includes a detailed three-way comparison of the genomes and deduced protein sets of B. bovis, Plasmodium falciparum and T. parva. An interesting conclusion from this comparison is that the number of nuclearencoded proteins targeted to the apicoplast (a plastid-like organelle of apicomplexans that is a potential target for antiparasite drugs) might be much smaller than previously believed. If true, this would have important consequences for future drug design. The knowledge of the genomic sequence will also boost the efforts towards creation of antibabesian vaccines. Genomes of the basidiomycetes Malassezia globosa and Malassezia restricta are certain to attract attention of those who suffer from dandruff, a nasty skin flaking condition that reportedly occurs in 30–95% of the human population (Xu et al., 2007). These fungi also cause a more serious skin disease, referred to as seborrheic dermatitis. Analysis of M. globosa genome revealed a large number of secreted hydrolases (phospholipases, aspartyl proteases), but an apparent absence of a fatty acid synthase gene. This observation correlated with the experimental data on lipid requirement for growth of M. globosa. A somewhat unexpected result of the phylogenetic analysis of M. globosa proteins was that its nearest neighbour is apparently a plant pathogen, corn smut fungus Ustilago maydis (Xu et al., 2007). Speaking about eukaryotic genomes, it might be appropriate to mention publication of the draft genome of the domestic cat Felis catus (Pontius et al., 2007). After all, this decidedly asocial eukaryote shares with us its home – and many microorganisms. Compared with the tiny – for eukaryotic organisms – genomes of B. bovis and M. globosa, the genome of the soil myxobacterium S. cellulosum looks like a monster: at more than 13 Mbp, it is almost as large and encodes as many proteins as those two combined (Schneiker et al., 2007). Furthermore, it is composed of a single chromosome, in contrast to the genome of B. bovis that consists of four chromosomes of 2.62, 2.59, 1.73 and 1.25 Mbp in length (Brayton et al., 2007), and the genome of M. globosa that apparently consists of eight even smaller chromosomes (Xu et al., 2007). The chromosome of S. cellulosum turned out to be even longer than the initial estimate of 12.2 Mbp (Pradella et al., 2002) and is currently the largest DNA molecule known in the prokaryotic world, far surpassing the previous record, the 10 Mbp chromosome of Solibacter usitatus. In the current classification, S. cellulosum belongs to the family Polyangiaceae in the order Myxococcales of the d-Proteobacteria. Myxococcus xanthus and Anaeromyxobacter dehalogenans, whose genomes have been sequenced earlier (Goldman et al., 2006), belong to a different family, Myxococcaceae. Stigmatella aurantiaca, *For correspondence. E-mail [email protected]; Tel. (+1) 301 435 5910; Fax (+1) 301 435 7793. Re-use of this article is permitted in accordance with the Creative Commons Deed, Attribution 2.5, which does not permit commercial exploitation. Environmental Microbiology (2008) 10(2), 281–288 doi:10.1111/j.1462-2920.2007.01552.x