A pyrG gene, encoding orotidine 5'-monophosphate decarboxylase, was cloned from a phage library derived from the lichen Solorina crocea. Phylogenetic analysis and a survey of geographically well-separated specimens were used to verify that the gene represented the fungal component of the lichen. Both coding and upstream sequences of S. crocea pyrG exhibited features typical of fungal genes. A 132-bp intron interrupting the coding region between nucleotides 157 and 288 was confirmed by RT-PCR and sequencing. Transformation of Aspergillus nidulans with S. crocea pyrG, controlled by either its native promoter or the A. nidulans trpC promoter, resulted in uridine-independent strains that exhibited appreciable growth only at 24 degrees C. Southern analysis indicated multiple integrations of S. crocea pyrG. These results demonstrate that heterologous expression may be used to investigate genes from lichens.