SILENT GENES IN BACTERIA - THE PREVIOUSLY DESIGNATED CRYPTIC ILVHI LOCUS OF SALMONELLA-TYPHIMURIUM LT2 IS ACTIVE IN NATURAL ISOLATES

Gene ilvG in Escherichia coli K-12 and ilvI in 'Salmonella typhimurium LT2' (S, enterica serotype Typhimurium, strain LT2) are inactive due to frameshift or nonsense mutations, respectively. These inactive gene s have been suggested to be part of 'cryptic' genetic systems which ar e defined as being of long-term regulatory and evolutionary significan ce. We have shown that the nonsense mutation in ilvI is present only i n derivatives of the laboratory strain 'S. typhimurium LT2'. All natur al isolates of Salmonella examined have an arginine codon at the corre sponding location of their ilvI sequences. Further, two randomly selec ted natural isolates of serotype Typhimurium are shown to each have an active ALS III isozyme. Our findings strongly suggest that the only S almonella strains which lack a functional ilvHI locus are LT2 isolates . We suggest that the mutations leading to inactivation of both ilvI i n 'S. typhimurium LT2' and ilvG in E. coli K-12 are more likely to hav e been acquired during laboratory storage and/or cultivation, rather t han representing cryptic systems of gene regulation.