Abstract

Expression of the arginase structural gene (agaA) in Aspergillus nidulans is subject to complex transcriptional and post-transcriptional regulation. Arginase mRNA has a long 5′-UTR sequence. Analysis of this sequence in silico revealed its putative complex secondary structure, the presence of arginine-binding motifs (arginine aptamers) and a short intron with two potential 3′ splicing sites. In this report we present evidence that L-arginine (i) binds directly to the arginase 5′-UTR; (ii) invokes drastic changes in the secondary structure of the 5′-UTR, unlike several other L-amino acids and D-arginine; and (iii) forces the selection of one of two 3′ splice sites of an intron present in the 5′-UTR. We postulate that expression of the eukaryotic structural gene coding for arginase in A. nidulans is regulated at the level of mRNA stability, depending on riboswitch-mediated alternative splicing of the 5′-UTR intron.