Lactobacillus exonuclease mutants: selection of mutants with altered ability to utilize deoxyribonucleotides

Lactobacillus acidophilus R-26, which requires preformed deoxyriboside for its growth, is able to grow in a basal medium supplemented with nitrophenyl esters of thymidine 3'- and 5'-phosphates. Mutants unable to grow on the thymidine phosphate esters were isolated and classified thus far into two classes. One class of mutants that are unable to grow on the diesters have modified exonucleases, their extracts being unable to hydrolyze the specific nitrophenyl ester on which the strain did not grow. It appears therefore, that the known 3'- and 5'-exonucleases specifically hydrolyzing these esters can be used for the support of growth. A second mutagenization treatment of one of the mutants gave rise to a new strain unable to use or hydrolyze either diester for growth, although it could still grow on several deoxyribodinucleotides. Another class of mutants does not grow on deoxyribooligonucleotides but grows on monodeoxyribo-nucleosides and -nucleotides. These mutants have normal levels of exonuclease activities, so it appears possible that they bear a mutation affecting permeability to diesters or to molecules larger than mononucleotides.