A TEMPERATURE-SENSITIVE DNA ADENINE METHYLTRANSFERASE MUTANT OF SALMONELLA-TYPHIMURIUM

A temperature-sensitive mutant of Salmonella typhimurium was isolated earlier after transposon mutagenesis with Tn10dTet. The mutant D220 gr ows well at 28 degrees C but has a lower growth rate and forms filamen ts at 37 degrees C. Transposon-flanking fragments of mutant D220 DNA w ere cloned and sequenced. The transposon was inserted in the dam gene between positions 803 and 804 (assigned allele number: dam-231::Tn10dT et) and resulted in a predicted ten-amino-acid-shorter Dam protein. Th e insertion created a stop codon that led to a truncated Dam protein w ith a temperature-sensitive phenotype. The insertion dam-231::Tn10dTet resulted in a dam ''leaky'' phenotype since methylated and unmethylat ed adenines in GATC sequences were present. In addition, the dam-231:: Tn10dTet insertion rendered dam mutants temperature-sensitive for grow th depending upon the genetic background of the S. typhimurium strain. The wild-type dam gene of S. typhimurium exhibited 82% identity with the Escherichia coli dam gene.