Figure 2

Frequency histogram of UP-UC cluster sizes in the E. coli metabolic network (grey bars). Data is shown in Supplementary Table S5 (see Additional File 1). The black line is the frequency distribution for the randomized versions of the network (averaged over 1000 realizations) that preserve the in- and out-degree of all nodes. Error bars show one standard deviation of the randomized ensemble. Inset: Enlargement of the graph for the larger sized clusters. In the real network, larger UP-UC clusters (size ≥ 8) occur much more often than in the randomized version (p < 0.001). On the other hand, smaller UP-UC clusters (size ≤ 3) occur much less often than in the randomized version (p < 0.001).