Fig. 5.

Functional categorization of transposon disrupted genes. Transposon disruption of Shigella genes with transport-related functions are more likely to be inferred as essential compared to clean deletions of similarly functioning genes in E. coli. We classified genes according to function using the MultiFun functional classification system [62]. For any category containing more than ten essential E. coli genes, we also calculated the number of Shigella-essential genes. As expected, most categories show a relative excess of Shigella-essential genes, as we inferred approximately 50 % more genes as being essential in Shigella versus E. coli (Fig. 3b). However, two functional categories show a clear excess above this level: substrate transport and active transport, showing a 3.2- and 2.2-fold increased probability of inferring a gene as being essential in Shigella as opposed to E. coli. This provides evidence that genes in these functional categories are more likely to be inferred as artefactually essential. This artefactual effect has not been noted previously. For each functional category (y-axis), we show the number of genes in that category (to the right of each bar); the number of genes found to be essential in E. coli (within each bar); and the level of enrichment of essential genes in Shigella (x-axis)