Microbial Mufanfs 53 



for an accessibility barrier, the indirect evidence they fur- 

 nish is strong. Studies on this aspect of cell physiology are 

 still in a primitive state; there is reason to hope that in in- 

 vestigation of this important area, mutants with altered ac- 

 cessibility will be of great help, just as auxotrophic mutants 

 have been in analyzing biosynthetic paths. To point out 

 how complex accessibility problems may be, I should like 

 to note that the barrier to DHQ just described has been 

 observed on the usual medium, in which glucose is the car- 

 bon source. If the glucose is replaced by xylose or succinate 

 the barrier disappears, and the primary auxotroph and its 

 secondary derivative then respond equally well to DHQ; 

 but under these conditions, addition of a trace of glucose 

 restores the barrier (12)! 



To bring the aromatic story up to date, I should like to 

 mention that a fusion of enzymatic and nutritional studies 

 with mutants has made it possible to trace this path farther 

 back than we were able to go by nutritional studies alone. 

 In this work, my colleague, Edwin Kalan, has recently 

 shown that mutants blocked before DHQ accumulate a 

 precursor, "compound V." This substance, though without 

 demonstrable growth-factor activity, could be recognized 

 through its enzymatic conversion to DHS by extracts of 

 mutants blocked after DHS. Furthermore, in collabora- 

 tion with David Sprinson of Columbia University, he 

 showed that these extracts also formed DHS from various 

 phosphorylated carbohydrates. The whole early part of the 

 path of aromatic biosynthesis is therefore now available for 

 enzymatic analysis. 



Finally, I should like to point out that the criteria pro- 

 posed for recognizing an essential intermediate are not re- 

 stricted to biosynthetic reactions. An extension to degrada- 

 tive reactions is useful, for though many of these reactions 

 have been exhaustively studied, it has not been possible, in 

 general, to assess definitively their physiological importance 



