An in planta induced gene of Phytophthora infestans codes for ubiquitin

An in planta induced gene of Phytophthora infestans (the causal organism of potato late blight) was selected from a genomic library by differential hybridization using labelled cDNA derived from poly(A)+ RNA of P. infestans grown in vitro and labelled cDNA made from potato-P. infestans interaction poly(A)+ RNA as probes. Sequence analysis showed that the gene codes for ubiquitin, a highly conserved protein which plays an important role in several cellular processes. The structure of the polyubiquitin gene (designated ubi3R) is consistent with the structure of other known polyubiquitin genes. It consists of three repeats in a head-to-tail arrangement without intervening sequences, each encoding a ubiquitin unit of 76 amino acids. The last ubiquitin unit is followed by an extra asparagine residue at the carboxy-terminal end. Northern and Southern blot analyses revealed that the polyubiquitin gene is a member of a multigene family, all genes of which show induced expression in planta.