SPONTANEOUS MUTATIONS IN PCAH AND PCAG, STRUCTURAL GENES FOR PROTOCATECHUATE 3,4-DIOXYGENASE IN ACINETOBACTER-CALCOACETICUS

Bacteria containing spontaneous null mutations in pcaH and -G, structu ral genes for protocatechuate 3,4-dioxygenase, were selected by exposu re of an Acinetobacter calcoaceticus strain to physiological condition s in which expression of the genes prevents growth. The parental bacte rial strain exhibits high competence for natural transformation, and t his procedure was used to characterize 94 independently isolated spont aneous mutations. Four of the mutations were caused by integration of a newly identified insertion sequence, IS1236. Many (22 of 94) of the mutations were lengthy deletions, the largest of which appeared to eli minate at least 17 kb of DNA containing most of the pca-qui-pob suprao peronic gene cluster. DNA sequence determination revealed that the end points of four smaller deletions (74 to 440 bp in length) contained DN A sequence repetitions aligned imprecisely with the sites of mutation. Analysis of direct and inverted DNA sequence repetitions associated w ith the sites of mutation suggested the existence of DNA slippage stru ctures that make unhybridized nucleotides particularly susceptible to mutation.