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Summary: Mutants were isolated from the non-capsulated strain Vollum of Bacillus anthracis which were unable to grow above 34° in the absence of certain pyrimidines. At elevated temperatures, one of the mutants, YC-TdR-, was found to be dependent on thymidine while the other, VC-T-, required thymine. Both mutants, however, grew normally in the absence of pyrimidines at near to room temperature. A relatively high concentration of thymine was needed in order to overcome the thymidine requirement of mutant YC-TdR- at 37°, whereas a combination of a low concentration of thymine with different deoxyribosides (deoxyadenosine, deoxyguano-sine, deoxycytidine) gave good growth of the mutant. This observation is suggestive of the presence of a particular enzyme, trans-N-deoxyribosylase. in the mutant VC-TdR-, an enzyme which appears to be of limited distribution in nature. The second mutant. VC-T-, utilized added thymine readily at 37° and the base could not be substituted by its nucleoside, thymidine. In fact, thymidine and deoxyribonucleosides inhibited the growth of mutant VC-T- in the presence of thymine.Both mutants also grew well at 37° in the presence of thymidine-5-phosphate, which indicated that the de novo pathway of pyrimidine synthesis is blocked above 34° somewhere in the pathway between deoxycytidine-phosphate and thymidine-5-phosphate. This block in the pyrimidine synthesis occurring at elevated temperatures caused an unbalanced synthesis of macromolecules accompanied by an abnormal cell-wall formation. At 37°, germinated spores showed an abnormal elongation of the initial cell concomitant with a gradual loss of viability. At this temperature cell-wall formation was also abnormal at limiting concentrations of pyrimidines and minor deficiencies in cell-wall structure of the mutants were still apparent even in the presence of a large excess of pyrimidine. This, however, did not involve any change in virulence of mutant VC+TdR- in the homoiothermic mouse.It is assumed that the mutants produce an altered enzyme protein corresponding to a block in de novo synthesis of pyrimidine, or that an inhibitor is produced at high temperatures which diminishes and finally prevents the action of the normal enzyme.

Temperature Sensitivity of Mutants of Bacillus anthracis Caused by a Block in Thymine-Nucleotide Synthesis