Chromosome mapping of the sweet potato little leaf phytoplasma reveals genome heterogeneity within the phytoplasmas

To further understand the genomic diversity and genetic architecture of phy toplasmas, a physical and genetic map of the sweet potato little leaf (SPLL ) strain V4 phytoplasma chromosome was determined. PFGE was used to determi ne the size of the SPLL-V4 genome, which was estimated to be 622 kb. A phys ical map was prepared by two-dimensional reciprocal digestions using the re striction endonucleases BssHII. SmaI. EagI and I-CeuI, Sixteen cleavage sit es were located on the map. Southern hybridizations of digested SPLL-V4 chr omosomal DNA were done using random clones and PCR-amplified genes as probe s. This confirmed fragment positions and located the two rRNA operons and t he linked fus/tuf genes encoding elongation factors G and Tu. respectively, on the physical map. An inversion of one of the rRNA operons was observed from hybridization data. Sequence analysis of one of the random clones iden tified a gid gene encoding a glucose-inhibited division protein. Digestions of the tomato big bud (TBB) phytoplasma chromosome with the same four enzy mes revealed genome heterogeneity when compared to the closely related SPLL -V4, and a preliminary chromosome size for the TBB phytoplasma of 662 kb wa s estimated. This mapping information has revealed that significant genome diversity exists within the phytoplasmas.