Characterization of the arom gene in Rhizoctonia solani, and transcription patterns under stable and induced hypovirulence conditions

The quinate pathway is induced by quinate in the wild-type virulent Rhizoctonia solani isolate Rhs 1AP but is constitutive in the hypovirulent, M2 dsRNA-containing isolate Rhs 1A1. Constitutive expression of the quinate pathway results in downregulation of the shikimate pathway, which includes the pentafunctional arom gene in Rhs 1A1. The arom gene has 5,323 bp including five introns as opposed to a single intron found in arom in ascomycetes. A 199-bp upstream sequence has a GC box, no TATAA box, but two GTATTAGA repeats. The largest arom transcript is 5,108 nucleotides long, excluding the poly(A) tail. It contains an open reading frame of 4,857 bases, coding for a putative 1,618-residue pentafunctional AROM protein. A Kozak sequence (GCGCCATGG) is present between +127 and +135. The 5'-end of the arom mRNA includes two nucleotides (UA) that are not found in the genomic sequence, and are probably added post-transcriptionally. Size and sequence heterogeneity were observed at both 5'- and 3'-end of the mRNA. Northern blot and suppression subtractive hybridization analyses showed that presence of a low amount of quinate, inducer of the quinate pathway, resulted in increased levels of arom mRNA, consistent with the compensation effect observed in ascomycetes.