Figure 1. Influence of introns on gene expression.

A. Schematic representation of the different cDNA molecules sequenced illustrating intron retention in CAS3 mRNA. CAS3 specific cDNA molecules were RT-PCR-amplified from purified mRNA, separated in three pools according to their sizes (Small, Medium and Large) and cloned. Fifteen clones were sequenced per pool and the presence of introns was analyzed. The numbers of clones obtained for each isoform in each pool are indicated. Introns and exons are not at real scale. B. cas3Δi is a non-functional allele. When the cas3Δ allele was replaced by the intronless allele, there were very little CAS3 mRNAs and no complementation of the capsule phenotype. (1) Antibody reactivity of the GXM-specific Mab CRND-8 with the mutant strains. 10⁴ cells were spotted on a nitrocellulose membrane and probed with CRND-8. (2) Northern blot experiment results showing very low mRNA level of the cas3Δi allele. C. Intron-dependent regulation of mRNA accumulation is gene specific. Two independently obtained Δi strains were tested here giving identical results (data not shown). As a control, we checked that replacing the cas3Δ allele by the wild type gene (CAS3) using the same procedure restored the antibody reactivity and the level of mRNA accumulation.