Fig 1. Visualization of the location of transposon insertion sites.

An image of the pst locus of S. meliloti generated using the Integrative Genomics Viewer [78]. Chromosomal nucleotide positions are indicated along the top of the image, and the location and relative abundance of transposon insertions are indicated by the red bars. Non-essential genes contain a high density of transposon insertions, whereas essential genes have few to no transposon insertions. Genes are color coded based on their fitness classification. The pstS, pstC, pstA, pstB, phoU, and phoB genes are co-transcribed as a single operon [92], and previous work demonstrated that non-polar phoU mutations are lethal in S. meliloti, whereas polar mutations are not lethal [93]. The lack of insertions within the phoU coding region is therefore consistent with the non-polar nature of the transposon.