Insertion element IS121 is near proA in the chromosomes of Escherichia coli K-12 strains

Abstract

Insertion element IS121 was mapped between proA and a previously mapped ISSA element in two F-prime plasmids. Results of hybridizations of IS121 to chromosomal DNA from four other strains suggest that IS121 is normally present at this position in the chromosomes of Escherichia coli K-12 strains.Insertion elements are responsible for several types of genome rearrangements that alter the sequence organization of the proA-purE region (-300 kilobase pairs [kb]) of the Escherichia coli K-12 chromosome. IS2 or IS3 elements mediate integration of F in this region (4), IS5 elements mediate type II F-prime excision (19), IS] elements are responsible for specialized transduction of argF by P1 (8,(20)(21)(22), and IS3 elements generate chromosomal inversions (2, 16). These events are reasonably described by recA-dependent recombination between pairs of insertion elements.There are features of the proA-purE region of the chromosome that are not explained by the mechanisms associated with the mapped copies of IS], IS2, IS3, and IS5 in the region. These features include the discontinuities in homologies between the proA-proC regions of E. coli K-12 and Salmonella typhimurium (3,11,14) and some categories of chromosomal deletions.Recently, a new insertion sequence, IS121 (possibly identical to IS30 [9]), has been reported as a component of the Mu dIl(Ap lac) bacteriophage (12). At least three copies of this 1.2-kb element appear in the chromosomes of several E. coli K-12 strains. Because of our interest in the causes for large chromosomal deletions in this region, we used an IS121 probe to screen F-prime plasmids for chromosomal IS121 copies lying in the chromosomal segment from 4 min (92 kb counterclockwise of proA) to the region of the lip gene near 14 min (-500 kb total) as well as an 11-kb region containing the aroG-gal genes (near 17 min). We determined that one IS121 copy is located in this region near proA (6 min).The AtraF proA+ plasmids pRH112 and pRH114 (derived from Hfr strains OR11 and OR72, respectively [5]), the F lac+ proC+ purE+ plasmid ORF4 (derived from Hfr OR11 [2]), and the F aroG+ gal' plasmid F152-1 (derived from Hfr P3 [13]) were digested with EcoRI, and the fragments were resolved electrophoretically and subjected to Southern hybridization analysis with 32P-labeled pMB0321 or pMB0301 as probe. Plasmid pMB0321 contains a BamHI fragment from pSC101::Mu dIl(Ap lac) and includes a copy of IS121. pMB0301 is similar to pMB0321, except that it contains no IS121 (12), and it served as a vector control in hybridization experiments. Plasmid ORF4, which carries chromosomal DNA from ISSA to lip inclusively (7,19), did not hybridize to pMB0321. Plasmid F152-1, which carries at least 14.2 kb of chromosomal sequence clockwise of a434 (near 12 min) * Corresponding author. and about 11 kb of DNA containing the aroG and gal genes (13,19), also failed to hybridize to pMB0321. However, the 5.8-kb EcoRI fragments and the 8.3-kb BamHI fragments from the AtraFproA' plasmids pRH112 (Fig. 1) and pRH114 (data not shown) hybridize...