Induction and repression of the histidine-degrading enzymes in Aerobacter aerogenes.

It has previously been shown that the enzymes that catalyze the first two steps, L-histidine ammonia-lyase and urocanase, are induced by L-histidine and repressed by catabolites (4, 5). It will be shown in the present paper that the enzyme catalyzing the fourth step of the sequence, N-formimino-r-glutamate formiminohydrolase, is also subject to induction by n-histidine and repression by catabolites. These observations raise the question whether the enzymes respond individually or as a unit to induction and repression. A possible approach to this problem suggested itself when it was found that the organism can grow not only in a medium containing histidine as the sole source of carbon, but also, though at a slower rate, in a medium containing urocanate as the sole source of carbon; this finding indicated that at least three of the enzymes of histidine degradation can also be induced by urocanate. It was therefore possible to investigate the effect of two different inducers, L-histidine and urocanate, on the formation of the enzymes of histidine degradation. Furthermore, the degree of catabolite repression could be varied by the use of different carbon compounds as major sources of energy in the presence and absence of ammonia (4-6). The results of this investigation show clearly that the first two enzymes respond in coordinate fashion to induction and repression, but that the response of the fourth enzyme is not coordinated with that of the first two enzymes.