Transcription of the Bz2 locus

--Gregor Schmitz, Nikolaus Theres and Peter Starlinger

The Bz2 gene is required for the development of full purple pigmentation in various tissues of the maize plant. Recessive mutations in the Bz2 gene lead to a bronze pigmentation of the aleurone layer and modify purple plant colour to reddish-brown. Inter-tissue complementation studies provide evidence that the Bz2 gene is involved in one of the last steps of the anthocyanin pathway but so far the gene product has not been characterized.

After cloning a DNA fragment from the bz2-m allele and a homologous fragment from a wild-type Bz2 line, we have used Bz2 specific probes to analyze the transcription pattern of this gene. Using different DNA fragments close to or spanning the Ds insertion site we were able to detect at least three different transcripts in RNAs from deeply pigmented leaf sheath and husk tissue (R B Pl plants). A transcript of about 1kb is detected with a single strand-specific probe located close to the Ds insertion site. When we use the opposite strand of the same DNA fragment as a probe, two larger transcripts of about 1.2kb and 2.7kb are detected. By comparing the strength of the hybridisation signals to known amounts of single stranded DNA we have estimated the relative abundance of the transcripts. The 1kb transcript was found to be about 100 times more abundant than the others.

A preliminary experiment suggests that the transcription of these RNAs in kernels depends on the action of the C1 locus (collaboration with Udo Wienand, Max-Planck-Institut fuer Zuechtungsforschung, Koeln). High amounts of all three transcripts were found in a line carrying a C1-s allele, whereas they are absent in RNA of a c1-m1 line without Ac.

The results of the Northern experiments could be confirmed by the analysis of cDNA clones. Most of the cDNA clones which were isolated from cDNA libraries prepared from leaf sheath and kernel tissue, are derived from the abundant 1kb transcript, two others are derived from a transcript of the opposite DNA strand. Sequence analysis revealed that the cDNA clones derived from the antisense transcripts partially overlap with the abundant 1kb transcript. From the sequence analysis we also conclude that the 1kb transcript codes for a protein; the function of the antisense transcripts is not known.

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