A NOVEL WOUND-INDUCIBLE EXTENSIN GENE IS EXPRESSED EARLY IN NEWLY ISOLATED PROTOPLASTS OF NICOTIANA-SYLVESTRIS

A cDNA clone (6PExt 1.2) encoding a novel extensin was isolated from a cDNA library made from 6 h old mesophyll protoplasts of Nicotiana syl vestris. The screening was performed with a heterologous probe from ca rrot. The encoded polypeptide showed features characteristic of hydrox yproline-rich glycoproteins such as Ser-(Pro)(4) repeats and a high co ntent in Tyr and Lys residues. The presence of four Tyr-X-Tyr-Lys moti fs suggests the possibility for intramolecular isodityrosine cross-lin ks whereas three Val-Tyr-Lys motifs map participate in intermolecular cross-links. The analysis of genomic DNA gel blots using both the N. s ylvestris and the carrot clones as probes showed that the 6PExt 1.2 ge ne belongs to a complex multigene family encoding extensin and extensi n-related polypeptides in N. sylvestris as well as in related Nicotian eae including a laboratory hybrid. This was confirmed by the analysis of RNA gel blots: a set of mRNAs ranging in size from 0.3 kb to 3.5 kb was found by the carrot extensin probe. The 6PExt 1.2 probe found a 1 .2 kb mRNA in protoplasts and in wounded tissues as well as a 0.9 kb m RNA which seemed to be stem-specific. The gene encoding 6PExt 1.2 was induced by wounding in protoplasts, in leaf strips and after Agrobacte rium tumefaciens infection of stems.