SEQUENCE DIVERGENCE OF THE MURB AND RRFB GENES FROM ESCHERICHIA-COLI AND SALMONELLA-TYPHIMURIUM

The murB gene of Salmonella typhimurium was cloned and found to be 75% and 82% identical to the DNA and protein sequences, respectively, of the same gene in Escherichia coli. These identities are among the lowe st recorded between the two bacteria. Nevertheless, wild-type S. typhi murium murB complemented the known temperature-sensitive E. coli mutan t, and wild-type E. coli murB complemented three temperature-sensitive mutants of S. typhimurium. The 5S rRNA gene, rrfB, and the region bet ween murB and rrfB were also cloned and sequenced. The rrfB gene of S. typhimurium differs from rrfB of E. coli in only 2 of 120 nt, but the region between murB and rrfB has diverged greatly and includes a sequ ence that closely resembles a repetitive extragenic palindrome of the type normally associated with E. coli. Previous comparisons of gene di vergence have suggested that the chromosomal mutation rate is lower in the vicinity of the origin of replication. However, the S. typhimuriu m murB gene, located 6 map minutes from the origin of replication, is highly substituted at synonymous sites and the sequence between murB a nd rrfB is significantly modified as well. Thus, murB is an exception to the general observation that genes near the origin of replication s how less divergence than do genes elsewhere in the bacterial chromosom e.