References

316 E.Davies,B.Stankovic
Relative mRNA accumulation (%)
160
140
120
100
80
60
40
20
I II
III
0
0 5 10 15 20 25
IV
Time after wounding (min)
Total mRNA
Polysomal mRNA
Fig.21.4. Wound-induced systemic accumulation of total and polysomal messenger RNA
(mRNA). Plants were wounded at zero time. Total and polysomal mRNA were isolated and
the general pattern for 12 different mRNAs (including calmodulin, Rubisco SS, chloroplast
mRNA-binding protein, bZIP DNA-binding protein) is shown. The four phases of transcript
accumulation are indicated with Roman numerals (Davies et al., unpublished results).
Note that the rapidly synthesized transcripts (phase 1) are degraded (phase 2) and not
used in protein synthesis. The slowly accumulating transcripts (phase 3) are recruited into
polysomes (phase 4) for protein synthesis
but then pauses to check for accuracy. However, when it is phosphorylated,
it “goes into overdrive” (Landick 1999) and keeps adding nucleotides
without checking for errors; thus, mRNA made by phosphorylated pol2
is virtually certain to be error-ridden. An alternative explanation for the
ultrarapid increase in synthesis of mRNA is that pol2 copies one strand,
then, rather than detaching and finding the appropriate start site on the
sense strand, reveres direction and copies the nonsense strand (Cornellissen
1989). We have used primers for the antisense mRNA, but have not
found any; thus, the most likely explanation for ultrarapid transcript accumulation
is through speeding up (activation, phosphorylation) of pol2.
This might take place via the same microfilament system used to phosphorylate
myosin and EF2, since it has been shown that actin filaments
penetrate directly into the nucleus and are associated with both pol2 and
active transcription sites (Hofmann et al. 2004).
Whatwouldbetheconsequencesofthecellmakingerror-riddenmRNA?
First, the ribosome would scan it for accuracy, find it in error and prevent
other ribosomes from attaching to it. Activated ribonucleases (Abler and
Green 1996; Gutierrez et al. 2002; LeBrasseur et al. 2002) would then degrade
it in a wonderfully orchestrated surveillance mechanism (Kozak