Interference 103,781
In order to increase and optimize the expression of
B.t.k. in plants it will be necessary to increase the
level of stable B.t.k. mRNA. In the case of tomato
such an increase would lead to higher levels of toxin
production and greater insecticidal activity. In the
case of tobacco no insecticidal activity can be expected
until chimeric genes have been constructed which permit
the production of stable mRNA. [(001088)]
. . . Instability of the B.t.k. mRNA could be due to its
unusual base composition. Genes from Bacillus species
typically have a base composition of greater than 60% A+T.
. . . By comparison the NPTII coding sequence from E. coli
whose RNA is expressed in plants at much higher levels
than B.t.k. is 47% A+T. Typically, plant coding sequences
are about 50% to 55% A+T. [(001088)]
It is possible that mRNAs which have a high A+T composition
are inherently unstable in plant cells either due to their
base composition per se or because they are unable to fold
into proper stabilizing structures. It is also possible
that the instability of these mRNAs is due to the presence
of specific oligonucleotide sequences. Possible functions
of such specific oligonucleotides which could lead to
instability of the transcribed mRNA include:
a. Specific signals for nucleolytic degradation of
the RNA.
b. Signals for improper polyadenylation of the RNA.
c. Premature termination of transcription.
d. Signals for improper splicing of the RNA.
Other functions for specific oligonucleotide sequences
leading to instability are also possible. [(001088)]
Specific signals rich in A+T are known to function in at
least two of the cases listed above. It has recently
shown by Shaw and Kamen [(Shaw, G. et al., R., Cell,
Vol. 46, pp. 659-667 (1986))] that a 51 nucleotide sequence
composed solely of A and T can cause a normally stable mRNA
to become very unstable. In addition they show that many
animal cell mRNAs known to be unstable contain A+T rich
sequences; these A+T rich sequences usually contain the
specific short oligonucleotide ATTTA. It is also known
that part of the signal for polyadenylation of mRNA is the
presence of a specific short oligonucleotide in the mRNA.
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