 The first complete DNA sequence of an E. coli genome ( laboratory strain K @-@ 12 derivative MG1655 ) was published in 1997 . It was found to be a circular DNA molecule 4 @.@ 6 million base pairs in length , containing <unk> annotated protein @-@ coding genes ( organized into <unk> operons ) , seven ribosomal RNA ( rRNA ) operons , and 86 transfer RNA ( tRNA ) genes . Despite having been the subject of intensive genetic analysis for about 40 years , a large number of these genes were previously unknown . The coding density was found to be very high , with a mean distance between genes of only 118 base pairs . The genome was observed to contain a significant number of <unk> genetic elements , repeat elements , cryptic <unk> , and bacteriophage remnants . 
