The sul gene, the gene specific suppressor of the lon gene, has been mapped and is located on the E. coli chromosome at approximately 22 minutes. Transductional studies have established a linkage of 31% between sul and pyrD and 82% between sul and fabA. A DNA repair deficient mutant, exrB, has been shown to have the following pleiotrophic properties in common with exrA (lex) mutants: suppression of UV induced mutation, rapid degradation of DNA following exposure to UV, reduction in recombination frequency and delayed induction of prophage. The significant difference between the two mutants is that exrB strains are temperature sensitive forming filaments at the nonpermissive temperature and exrB lon strains spontaneously filament at temperatures permissive for growth. Both exrA (lex) and exrB are linked to the malB locus but have separate chromosomal locations.