Age distribution of horizontally transferred DNA in the E. coli MG1655 chromosome. Bars represent the amount of protein-coding, horizontally transferred DNA of the specified age present in the 571 genes analyzed by back amelioration. The remaining 184 genes could not be successfully analyzed because of aberrant nucleotide compositions, which could result from horizontal transfer among more than one genome; most of the genes comprising this group are encoded by prophages and mobile genetic elements.

Distribution of horizontally transferred DNA in the E. coli MG1655 chromosome. Within each centisome, each bar denotes a continuous segment of transferred DNA containing one or more ORFs; and the length of each bar represents its size rounded to the nearest 500 bp. Features of transferred regions, such as duration in the chromosome and the identification of repeated and mobile elements, follow the notation presented in the key. The age of each continuous segment of DNA was inferred from the ages of genes successfully analyzed by back-amelioration; segments lacking genes of known age are shown in black, and no segment comprised genes with significantly different ages. Positions of the replication origin (oriC) and terminus (terC), as well as the identity of the specific tRNA loci found to be adjacent to a horizontally transferred region, are noted on the left of the open bar representing the MG1655 chromosome. The nomenclature for phage and IS elements, and for genes of known function contained within a particular transferred segment, are shown within the corresponding bar. The identities of insertion sequences are noted except as follows: adjacent IS911/(fragment)/IS3 are located within minute 5; adjacent IS3/IS600 are located within minute 8; and adjacent IS2/IS30 are located within minute 31.