The very nature of the parasitic lifestyle enables these organisms to substantially reduce the size of their genome. In 22 November Nature, Michaël Katanika and colleagues at Genoscope, describe the genome of the microsporidian parasite Encephalitozoon cuniculi, which seems to have taken this reduction process to the very limit (Nature 2001, 414:450-453.).

The 2.9 Mb E. cuniculi genome consists of 11 chromosomes with 1,997 potential protein coding sequences tightly packed together with little 'junk' DNA in between. It has long been thought that the microsporidia diverged evolutionarily before the development of mitochondria, as these organelles are absent from the cytoplasm of the parasite. But, with the completion of the genome sequence 5 proteins classically associated with Fe-S cluster assembly machinery — an essential function of mitochondria — have been identified. It is still unclear if these genes have relocated to a site within the cytoplasm...