A very poorly transformable mutation at the threonine deaminase locus in Bacillu subtilis

Summary An isoleucine requiring mutant of Bacillus subtilis which displays some novel properties has been studied. The strain lacks threonine deaminase activity and is also more sensitive than the wild type to the action of mitomycin C without being more sensitive to UV light. The two features (isoleucine requirement and mitomycin sensitivity) are transferred simultaneously in transformation. The isoleucine marker of this strain is situated at the right end in the map of the threonine deaminase locus. It is at present the nearest known marker to the terminus of the B. subtilis chromosome. Relative transformation frequency for this marker is very low suggesting a low integration efficiency. This is true for both alleles (wild type and mutant) of the marker. SP 10 transduction frequency is almost nil whereas PBS-1 transduction is as effective as for any other marker in that region. Any treatment which causes breakage of the strands in wild type DNA (dilution, shearing, deoxyribonuclease I action) brings about a very important decrease in the relative frequency of transformation for this marker. The marker is more sensitive to these treatments than the weak linkage of two very distant markers. Results agree with the hypothesis that the mutation in this strain corresponds to a deletion in the terminal region of the chromosome. The deletion (suggested also by the absence of reversion) should nevertheless be very long, probably exceeding the average length of a transforming DNA segment. However, an alternative hypothesis has been considered and discussed.