The expression of the Arabidopsis thaliana PAT1 gene, which encodes the tryptophan biosynthetic enzyme phosphoribosylanthranilate transferase, was investigated using translational fusions of the PAT1 promoter to the GUS reporter gene. Independent stably transformed A. thaliana lines containing a single copy of a fusion that includes the entire plastid transit peptide and the first two introns of PAT1 had on average 30 times more GUS enzyme activity than plants transformed with a construct in which GUS was fused a short distance downstream of the PAT1 start codon. Plants containing the construct without introns or leader peptide accumulated undetectable amounts of PAT1-GUS fusion protein and mRNA, even though the transcriptional rate of both fusion constructs was comparable. Fusions containing the entire transit peptide and either of the first two introns yield as much GUS activity as constructs containing both introns, but constructs containing the transit peptide but no introns give rise to much lower levels. Therefore, introns greatly enhance the expression of PAT1-GUS fusions, and they act post-transcriptionally to increase the steady-state level of mRNA.