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J. Bacteriol., 11 1997, 6824-6830, Vol 179, No. 21
CJ Wu and GR Janssen
The chloramphenicol acetyltransferase (cat) gene from Streptomyces
acrimycini encodes a leaderless mRNA. Expression of the cat coding sequence
as a leaderless mRNA from a modified lac promoter resulted in
chloramphenicol resistance in Escherichia coli. Transcript mapping with
nuclease S1 confirmed that the 5' end of the cat message initiated at the A
of the AUG translational start codon. Site-directed mutagenesis of the lac
promoter or the cat start codon abolished chloramphenicol resistance,
indicating that E. coli initiated translation at the 5' terminal AUG of the
cat leaderless mRNA. Addition of 5'-AUGC-3' to the 5' end of the cat mRNA
resulted in translation occurring also from the reading frame defined by
the added AUG triplet, suggesting that a 5'- terminal start codon is an
important recognition feature for initiation and establishing reading frame
during translation of leaderless mRNA. Addition of an untranslated leader
and Shine-Dalgarno sequence to the cat coding sequence increased cat
expression in a cat:lacZ fusion; however, the level of expression was
significantly lower than when a fragment of the bacteriophage lambda cI
gene, also encoding a leaderless mRNA, was fused to lacZ. These results
indicate that in the absence of an untranslated leader and Shine-Dalgarno
sequence, the streptomycete cat mRNA is translated by E. coli; however, the
cat translation signals, or other features of the cat mRNA, provide for
only a low level of expression in E. coli.
Copyright © 1997, American Society for Microbiology
Expression of a streptomycete leaderless mRNA encoding chloramphenicol acetyltransferase in Escherichia coli
Department of Microbiology, Miami University, Oxford, Ohio 45056, USA.
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