Control Sequences of Different Plants -

By comparing DNA, sequence data for several different plant genes it is possible to identify a number of conserved regions which, by analogy with animal systems, seem likely to be important for accurate transcription and processing of RNA. Beginning at the 3' (downstream) end and working backward, one first encounters the poly (A) addition signals.

Poly (A) is added posttranscriptionally to most eukaryotic mRNAs. In most animal systems it has been shown that a short sequence near the 3'-end of ' the mRNA contains the information necessary for proper 3'-end processing and poly(A) addition. This sequence is AATAAA. It is located at about 10-33 bases upstream from the poly (A) tail.

Is the structure of the 3'-end of plant genes similar to animal or yeast genes? The animal consensus sequence AATAAA has been found in all the plant genes of the zein multigene family examined so far, with the exception of the 849 subfamily of zein. This subfamily, which includes Z7, 849, and pZ22.3, has the sequence AATAAT instead of the normal AATAAA.

Even though most plant genes have AATAAA, its location with respect of the poly(A) tail differs from that in animal genes. In all the leg hemoglobin genes, most of the zein genes and the alcohol dehydrogenase genes of maize the AATAAA is closer to the stop codon of the protein coding sequences than to their poly (A) tail.

The leghemoglobin genes have GATAAA and the alcohol dehydrogenase genes AATGAG. In contrast, AATAAG in the legumin gene is near the stop codon and AATAAA near the, poly (A) tail. Multiple adenylation signals occur more often in plant genes than in animal genes, in which only a few multiple polyadenylation sites have been observed.

Hence the resultant 3'-RNA fragment is metabolically unstable and difficult to study. Be that as it may, the existence of an elaborate 3'-processing system may make the actual termination process less crucial since even transcripts whose 3'-ends are not well defined can be cleared and polyadenylated to produce mRNA molecules with the correct terminal sequences.

Within the coding region plant genes do not seem to differ much from animal genes. Translation stop and intron splicing signals are similar and the initiator codon (ATG in DNA, AUG in RNA) occurs within a consensus sequence very similar to its counterpart in animal genes. Most of the transcriptional control sequences are located in the region 5' to .the start of transcription.

The position of the translation start codon is not a good guide to the position at which transcription starts, however, since transcribed but untranslated sequences of variable length are commonly found between these two positions.