Reduced expression of a distal gene of the prn gene cluster in deletion mutants of Aspergillus nidulans: genetic evidence for a dicistronic messenger in an eukaryote.

The prn gene cluster involved in L-proline catabolism in Aspergillus nidulans, has the gene order prnA-prnD-regulatory region-prnB-prnC. prnB, prnD, and prnC specify proline permease, proline oxidase, and delta1-pyrroline-5-carboxylate (P5C) dehydrogenase, respectively. prnA is probably a positive regulatory gene whose product is necessary for expression of the prn activities. Proline induces proline permease and P5C dehydrogenase in prnD- mutants which lack proline oxidase, showing that proline does not have to be converted to P5C to act as inducer. Deletion mutations extending from within prnD to within prnB result in considerably reduced expression of prnC, whereas a prnD- prnB- double mutant shows normal prnC expression. This strongly suggests that the deletion mutations eliminate a promotor/initiator site for transcription of a dicistronic messenger for prnB and prnC. The fact that the deletions do not eliminate prnC expression altogether indicates that at least one other species of prnC transcript (monocistronic, tricistronic, or tetracistronic) can be made.