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Summary: Transductional analysis was applied to the Pseudomonas aeruginosa mutant pa014 (hnc-1). This mutant can utilize l-histidinol as sole source of carbon and nitrogen and has a 60-fold increased histidinol dehydrogenase (HDH) content ( Dhawale, Creaser & Loper, 1972 ). Transductional analysis was carried out using 18 histidine-requiring mutants to see where the hnc-1 locus maps in relation to the structural genes of histidine biosynthesis. The hnc-1 marker cotransduced with group IV genes at 97 to 100 % and not at all with group I, which is known to be the structural gene for HDH.
The data obtained in the studies of K m (histidinol) and K m (NAD), and the effect of pH and temperature on the HDH activity from pa01 and pa014 are in full agreement with the genetic data that the hnc-1 mutation is not in the structural gene for HDH. It is suggested that hnc-1 may be a mutation in a regulatory gene affecting HDH synthesis in pa014 and may map close to his-IV whose function in histidine biosynthesis is not known.