Characterization of Three Related Low-Temperature-Regulated cDNAs from Winter Brassica napus
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A cDNA clone, pBN115, encoding a low-temperature-regulated transcript in winter Brassica napus has been isolated. Northern blot analyses show that levels of transcripts hybridizing to pBN115 increase within 24 h of exposure of B. napus to low temperature, peak at 3 d, and then remain at an elevated level for the duration of the cold treatment (up to 10 weeks). Transferring plants from 2 degrees C to room temperature results in the loss of detectable transcripts hybridizing to pBN115 within 1 d. The transcript was not detected in RNA isolated from roots of cold-acclimated B. napus. Results of in vivo labeling of nascent RNA in leaf discs of B. napus with thiouridine suggest that regulation of expression may be transcriptional, at least at the onset of cold temperature. Although drought stress leads to a slight increase in transcript level at room temperature, neither a brief exposure to elevated temperatures nor exogenous application of abscisic acid resulted in the appearance of the transcript represented by pBN115. Furthermore, transcripts hybridizing to pBN115 were present at the same levels whether the plants were acclimated in the light or dark. Hybridization experiments show that pBN115 hybridizes strongly to cold-regulated transcripts in Arabidopsis thaliana, Descurania sophia, and spring B. napus, all of which are cruciferous plants capable of cold acclimation. No hybridizing transcript could be detected in cold-acclimated Spinacea oleracea, winter Secale cereale, or cold-grown Nicotiana tabacum. DNA sequence analysis of pBN115 reveals a single open reading frame that potentially encodes a protein of 14.8 kD. This size closely approximates that of a polypeptide produced by in vitro transcription/translation experiments. Two additional cDNA clones, pBN19 and pBN26, with divergent 5'-and 3'-untranslated regions, were also isolated and found to encode similar, but not identical, polypeptides.
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