--D. Gupta, M. Tanzer and A. G. Abbott
In order to study transcriptional patterns in higher plants, cloned probes were used for hybridization on Northern blots. These studies revealed the presence of transcripts homologous to the bacterial plasmid pBR322. This homology was observed in both mitochondria and whole cell RNA extracts. The pBR322-homologous transcripts appear to be in a higher copy number in the mitochondrial RNA as compared to the whole cell RNA. These transcripts have been identified in several different strains. The plasmid pBR322 when used as a probe hybridizes to two major transcripts that are approximately 3400 bases and 1800 bases in size. RNA from different tissues of the plant was also used in Northern blot analyses to study pBR322 homology. These studies revealed differences in transcript pattern between coleoptile and stem tissue. The larger transcript is present in a lower copy number in the stem tissue as compared to the coleoptile tissue. This difference in transcripts may be important as a developmental regulation mechanism.
The next set of experiments was designed to identify genes that encode for the pBR322 homologous transcripts. A pBR322 probe was hybridized to a Southern blot with mitochondrial and genomic DNA. DNA fragments homologous to pBR322 were identified in both the samples of DNA. The major homology to pBR322 lies on two HindIII fragments, 6.0kb and 3.2kb in size, in a strain of B73N. This homology is observed only in DNA preparations that are not run on a CsCl gradient. This suggests that pBR322 homologous DNA has a GC ratio different from the GC ratio of the genomic DNA. Other strains have also been studied. A variation in the hybridization pattern among these different lines is observed.
We do not know if these DNA molecules encode the transcripts that hybridize
to pBR322. Experiments are currently being done to investigate this.
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