Molecular cloning and analysis of abundant and stage-specific mRNAs from Puccinia graminis

To characterize highly expressed mRNAs from germinated urediniospores of Puccinia graminis f. sp. tritici, we isolated 68 cDNA clones of abundant mRNA species belonging to at least six homology groups. The two most abundant homology groups, HG1 and HG2, contained 54 of the 68 cDNA clones and accounted for 2.4 and 0.6% of the poly(A)+ RNA in germinated urediniospores, respectively. By sampling different developmental stages of the uredinial cycle, we showed that the uam transcript, corresponding to HG2, accumulated in all stages of hyphal and urediniospore development, whereas the accumulation of usp transcript, corresponding to HG1, was specific to the sporulation stage. Southern blot analysis indicated that usp is a small gene family consisting of three to four members. Sequence analysis of 10 cDNA clones indicated that two different members of the usp gene family were expressed in germinated urediniospores. This gene family encodes small hydrophobic polypeptides of 113 amino acids with an unusual amino acid composition, in that alanine, glycine, leucine, and proline represent 48% of the protein. These polypeptides are predicted to be localized extracellular because they contain a putative signal sequence and may be functionally related to hydrophobins, a family of small hydrophobic proteins abundantly expressed during sporulation in Schizophyllum commune and Aspergillus nidulans. The uam and usp genes deserve further investigation, including isolation of genomic clones. The regulatory regions of the uam gene, which is highly expressed in hyphae, may be useful in the construction of a transformation vector for rust fungi.