Ubiquitin in a lower plant. Characterization of ubiquitin-encoding DNA and RNA from Chlamydomonas reinhardii.

A detailed characterization of Chlamydomonas reinhardii cDNAs encoding ubiquitin 52-amino-acid fusion proteins is presented in this study. While two cDNAs (designated UBI1 and UBI3) encode the complete ubiquitin extension protein, the third one (UBI2) lacks a minor part of the 5' region as well as a poly(A) tail. Differences between UBI1 and UBI3 are observed in the length of the poly(A) domain (13 versus 46 adenines) and in the lack of three nucleotides at the 3' noncoding region of UBI3. According to Northern blot experiments using UBI1 as a homologous probe, at least six members of the C. reinhardii ubiquitin gene family are transcriptionally active at regular conditions. During application of severe stress (heat shock in light and darkness, and photoinhibition), the transcription of the UBI1 mRNA substantially decreases. This effect is most drastically induced by application of heat shock to illuminated cells.