engleski

Linear and circular forms of PMUs (potential mobile units) in the Aster Yellows phytoplasma strain Witches Broom (AY-WB) genome and their role in adaptation to plant and insect hosts

Phytoplasmas are insect-transmitted plant pathogenic bacteria that continuously cycle back and forth between their plant and insect hosts. These bacteria have a reduced genome that lack genes for basic metabolic pathways making these bacteria recalcitrant to in vitro cultivation so far. However, the chromosomes contain numerous large repeated units of ~ 20 kb in size that resemble integrative and conjugative elements (ICEs). These units were named potential mobile units (PMUs) (Bai et al. 2006). Their abundance and conserved gene order in the phytoplasma chromosomes lead to the hypothesis that they have a positive contribution to phytoplasma fitness (Bai et al. 2006). Here we demonstrate that PMU1 of the Aster Yellows phytoplasma strain Witches Broom (AY-WB) exists as linear (L-PMU1) and circular (C-PMU1) forms of which the latter is more abundant during insect infection. All genes encoded in PMU1, except the insertion sequences tra5, are more abundantly expressed in insects relatively to plants. This corresponds to the presence of predicted promoter sequences upstream of the first gene, sigF, in C-PMU1 but not in L-PMU1. Thus, PMUs are apparently part of a phase-variation mechanism allowing phytoplasma adaptation to their insect hosts.

Structural evolution; genome stability/plasticity; inverted repeats; replicative composite transposon; ‘ Ca. Phytoplasma asteris’

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