ABSTRACT
Phytoplasmas
(“Candidatus Phytoplasma,” class
Mollicutes) cause disease in hundreds of economically
important plants and are obligately transmitted by sap-feeding insects
of the order Hemiptera, mainly leafhoppers and psyllids. The 706,569-bp
chromosome and four plasmids of aster yellows phytoplasma strain
witches' broom (AY-WB) were sequenced and compared to the onion yellows
phytoplasma strain M (OY-M) genome. The phytoplasmas have small
repeat-rich genomes. This comparative analysis revealed that the
repeated DNAs are organized into large clusters of potential mobile
units (PMUs), which contain tra5 insertion
sequences (ISs) and genes for specialized sigma factors and membrane
proteins. So far, these PMUs appear to be unique to phytoplasmas.
Compared to mycoplasmas, phytoplasmas lack several recombination and
DNA modification functions, and therefore, phytoplasmas may use
different mechanisms of recombination, likely involving PMUs, for the
creation of variability, allowing phytoplasmas to adjust to the diverse
environments of plants and insects. The irregular GC skews and the
presence of ISs and large repeated sequences in the AY-WB and OY-M
genomes are indicative of high genomic plasticity. Nevertheless,
segments of ∼250 kb located between the lplA and
glnQ genes are syntenic between the two phytoplasmas and
contain the majority of the metabolic genes and no ISs. AY-WB appears
to be further along in the reductive evolution process than OY-M. The
AY-WB genome is ∼154 kb smaller than the OY-M genome, primarily
as a result of fewer multicopy sequences, including PMUs. Furthermore,
AY-WB lacks genes that are truncated and are part of incomplete
pathways in OY-M.