Pathogenic bacteria with highly reduced genomes

Fitoplazme su bakterije koje nemaju staničnu stjenku i zbog toga pripadaju razredu Mollicutes. Ovi patogeni su intracelularni paraziti biljnog floema, a prenose se kukcima koji im služe i kao domaćini. Dok kod kukaca najčešće ne izazivaju nikakve smetnje, učinak fitoplazmi na biljne vrste je često poguban. Najčešći simptomi su filodija, virescencija, rast u obliku vještičje metle, crvenjenje i žućenje listova, nekroza floema. Poznavanje biologije ovih specifičnih organizama je dosta ograničeno zbog toga što se ne mogu kultivirati na hranjivim podlogama. No u posljednje vrijeme počelo se s potpunim sekvenciranjem genoma fitoplazmi, što je dovelo do brojnih novih spoznaja. Imaju iznimno reducirane genome (680 – 1600 kb). Nedostaju im mnogi esencijalni geni u različitim metaboličkim putovima, a najznačajniji nedostatak su geni za sve podjedinice ATP-sintaze, kao i geni za uzimanje i iskorištavanje šećera. To nam ukazuje da fitoplazme aktivno unose metabolite domaćina. No mnogi od tih mehanizama unosa i putova iskorištavanja domaćinskih metabolita nisu poznati kao i onaj o uzimanju ATP-a. Usprkos tome svemu fitoplazme vrlo uspješno preživljavaju i manipuliraju domaćinima iz dva različita carstva. Smatra se da su se tako dobro prilagodile na takav način života zbog plastičnosti genoma. Fitoplazme su kompenzirale svoja ograničenja horizontalnim transferom gena, rearanžmanima DNA i rekombinacijama između kromosoma i PMUs (Potential Mobile Units). Upravo ti PMUs sadrže gene koji su važni za preživljavanje i prilagodbu na različite domaćine. Za sada je najbolji način kojim možemo doći do novih spoznaja o fitoplazmama daljnje sekvenciranje fitoplazmatskih genoma i njihove komparativne analize.Phytoplasmas are bacteria without cell wall belonging to the class Mollicutes. These pathogens are intracellular parasites of plant phloem, transmitted by insects which also serve as their hosts. Phytoplasmas do not cause any disturbance to most insect hosts but their infections of plants is often fatal to this host type. The most common phytplasmoses symptoms in plants are phyllody, virescence, witches broom, reddening or yellowing of leaves, necrosis of the phloem. Knowledge of the biology of these specific organisms is quite limited because they cannot be cultivated in culture medium. Neverheless, in recent time be the complete phytoplasma genome sequencing led to many new insights. Phytoplasmas have an extremely reduced genome (680 to 1600 kb). They lack many essential genes in different metabolic pathways, and most significantly, they can live with the lack of genes for ATP synthase subunits, as well as genes for sugar uptake and usage. It suggests that phytoplasmas effivciently uptake metabolites from the host. Hovewer, many of these uptake mechanisms are including the one for ATP. In spite of everything, phytoplasmas successfully survive and manipulate the hosts from two different kingdoms. It is believed that they have been so well adapted to that way of life because of their genome plasticity. Phytoplasmas compensated their genomic limitations by horizontal gene transfer, rearrangements of DNA and recombination between chromosomes and Potential Mobile Units (PMUs). PMUs contain genes that are important for survival and adaptation to different hosts. For the time being, one of the best ways to gain more knowledge on phytoplasmas is sequencing of additional phytoplasma genomes and comparative genome analysis