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Journal of Bacteriology, December 2002, p. 6730-6733, Vol. 184, No. 23
0021-9193/02/$04.00+0 DOI: 10.1128/JB.184.23.6730-6733.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Leaderless mRNAs Bind 70S Ribosomes More Strongly than 30S Ribosomal Subunits in Escherichia coli
Sean M. O'Donnell
and Gary R. Janssen*
Department of Microbiology, Miami University, Oxford, Ohio 45056
Received 15 March 2002/
Accepted 3 September 2002

ABSTRACT
By primer extension inhibition assays, 70S ribosomes bound with
higher affinity, or stability, than did 30S subunits to leaderless
mRNAs containing AUG or GUG start codons. Addition of translation
initiation factors affected ribosome binding to leaderless mRNAs.
Our results suggest that translation of leaderless mRNAs might
initiate through a pathway involving 70S ribosomes or 30S subunits
lacking IF3.

TEXT
Initiation of protein synthesis in prokaryotes involves several
steps that include the binding of a 30S ribosomal subunit to
the mRNA's ribosome binding site and placement of the start
codon in the ribosomal P site. Translation signals within the
mRNA, including the start codon (
16,
20,
25) and complementary
pairing between the mRNA Shine-Dalgarno (SD) and 16S rRNA anti-SD
sequences (
8,
9,
21), contribute to the efficiency and stability
of the translation initiation complex.
The efficiency and fidelity with which the initiation complex forms is influenced also by three initiation factors (IF1, IF2, and IF3) (6). IF1 enhances the activities of IF2 and IF3 (6) and is proposed to block binding of aminoacyl-tRNA to the 30S subunit A site during initiation (2, 14). IF2 promotes binding of the initiator tRNA at the P site of the 30S subunit (6). IF3 is proposed to have several functions, including the selection of initiator tRNA and discrimination against unusual start codons (17) and binding to 30S subunits to prevent joining to a 50S subunit (13).
Much of our understanding of translation initiation results from studies of canonical mRNAs containing a 5' untranslated leader and SD sequence. However, several leaderless mRNAs from the archaea, bacteria, and eucarya lack these features (reviewed in reference 26) but are still highly translated. It is not clear how leaderless mRNAs bind ribosomes, but several lines of evidence implicate a modified initiation pathway different from that observed with mRNAs containing a 5' untranslated leader and SD sequence. For example, 30S subunits containing IF3 discriminate against initiation at a 5' AUG initiation codon (4, 15, 24). Also, 30S subunits containing IF2 stimulate ribosome binding and translation of leaderless mRNAs (1, 4, 5). These data, and a report that 70S ribosomes bind leaderless mRNAs in vitro (1), suggest the possibility of a novel pathway for translation of leaderless mRNAs.
Leaderless mRNAs bind 70S ribosomes more efficiently, or more stably, than 30S ribosomal subunits.
Primer extension inhibition (toeprint) assays (7) were performed to investigate the relative ribosome binding strengths of leaderless mRNAs encoded by the Escherichia coli
cI, Tn1721 tetR, and P2 V genes. Ribosomes (70S) and 30S subunits were prepared from E. coli MRE600 as described previously (23). Toeprint assays and construction of cI-lacZ fusions were performed as described previously (12, 16). P2 gene V codons 1 to 16 from pGCV22 (3) were fused to lacZ and used as a template to generate V-lacZ leaderless mRNAs in vitro. Tn1721 tetR codons 1 to 16 (10) were constructed from oligonucleotides, fused to lacZ, and used as a template to generate tetR-lacZ leaderless mRNAs in vitro. Ribosome binding efficiencies for cI-lacZ mRNAs were quantified, with a Molecular Dynamics (Storm 800) phosphorimager, by taking the average from three independent toeprint assays and were expressed as toeprint signal/(full-length signal + toeprint signal).
A ternary complex (i.e., mRNA, ribosome, and initiator tRNA)-dependent toeprint signal was observed at position +16 relative to the start codon of leaderless cI-lacZ mRNA (Fig. 1A and B). Phosphorimager analyses revealed that the signal with 70S ribosomes was, on average, 18-fold stronger than the signal obtained with 30S subunits. Also, maximal toeprint signal intensity was observed when the 70S ribosome/mRNA ratio was 1:1 (Fig. 1B, lanes 2 and 3), while signal intensities have been observed to continue increasing beyond a 12-fold excess of 30S subunits over mRNA (data not shown). By using gene V and tetR leaderless mRNAs, a much stronger toeprint signal was observed with 70S ribosomes (Fig. 1C, lanes 1 and 2; Fig. 1D, lanes 3 and 4) than with 30S subunits (Fig. 1C, lanes 3 and 4; Fig. 1D, lanes 1 and 2). These results are consistent with an earlier report (1) and suggest that leaderless mRNAs have a higher affinity for, or are intrinsically more stable with, 70S ribosomes than 30S subunits.
70S ribosomes bind to leaderless cI mRNA with either an AUG or a GUG start codon.
Previous reports (
16,
25) indicate that leaderless mRNAs with
an AUG start codon are most efficiently expressed but that mRNAs
with a 5'-terminal GUG are also translated. Because 30S subunits
have been reported previously not to bind leaderless
cI mRNA
with a GUG start codon (
16), we investigated the ability of
70S ribosomes to bind this mRNA. With the use of
cI-
lacZ leaderless
mRNAs with AUG or GUG start codons (
16), 30S subunits yielded
the expected toeprint signal from mRNA with an AUG start codon
(Fig.
2A, lanes 3 and 4) but not from mRNA with a GUG start
codon (Fig.
2B, lanes 3 and 4); assays with 70S ribosomes revealed
a ternary complex-dependent signal for leaderless mRNA with
an AUG (Fig.
2A, lanes 1 and 2) or GUG (Fig.
2B, lanes 1 and
2) start codon. Toeprint reactions with
cI-
lacZ leaderless mRNAs
containing a UUG or CUG initiation codon did not reveal a toeprint
signal with 30S subunits (
16) or 70S ribosomes (data not presented).
These results suggest that leaderless GUG-initiated mRNAs have
substantially reduced affinity for, or do not associate stably
with, 30S subunits.
IFs affect 30S subunit binding to leaderless mRNAs.
The contribution of IFs in 30S subunit binding to leaderless
mRNAs was assessed by toeprint assays with
cI, gene
V, and
tetR leaderless mRNAs. IF1, IF2, and IF3 were isolated essentially
as described previously (
22) from IF-overproducing strains of
E. coli (kindly provided by C. Gualerzi). IFs were present in
a twofold molar excess over 30S subunits and were prebound to
30S subunits (37°C, 5 min) prior to addition of mRNA. Initiator
tRNA was charged and formylated, as described previously (
19).
Addition of IF1 reduced the toeprint signal for
tetR (Fig.
3C,
lanes 1 and 2) but did not significantly affect signal intensities
for
cI (Fig.
3A, lanes 1 and 2) or gene
V (Fig.
3B, lanes 1
and 2); however, addition of IF2 increased binding to each leaderless
mRNA (compare lanes 1 and 3 of Fig.
3A to C, respectively).
IF2 stimulation of 30S subunit binding to leaderless mRNA is
consistent with previous reports (
1,
4,
5). Addition of IF3
decreased toeprint signal intensity (compare lanes 1 and 4 of
Fig.
3A to C), consistent with an earlier report (
24). Addition
of IF1 and IF2 did not significantly affect toeprint intensity
relative to that with reactions with IF2 alone (compare lanes
3 and 5 of Fig.
3A to C). Addition of IF2 and IF3 (lane 6 of
Fig.
3A to C) or IF1 and IF3 (lane 7 of Fig.
3A and B) reduced
the toeprint signal, similar to that seen with IF3 alone, suggesting
that the inhibitory effect of IF3 dominates the stimulatory
effect of IF2. Interestingly, addition of IF1, IF2, and IF3
to 30S subunits eliminated the toeprint signal from all three
leaderless mRNAs (Fig.
3A, lane 8; Fig.
3B, lane 7; and Fig.
3C, lane 8), suggesting that the IFs disallow 30S initiation
complex formation on leaderless mRNAs; alternatively, IFs might
destabilize the ternary complex for detection by reverse transcription.
Based on the intracellular concentrations of ribosomes (

10 µM)
and IFs (

1 µM each), and IF
Kas for 30S subunits, Gualerzi
and Pon (
6) have speculated that most IFs in vivo are bound
to 30S subunits. Based on our toeprint results, this would suggest
that 30S subunits containing all three IFs are biased against
initiation complex formation with leaderless mRNAs. These data
further support the suggestion that 70S ribosomes, or 30S subunits
lacking a full complement of IFs, might bind and initiate translation
of leaderless mRNAs and explain the way by which leaderless
mRNAs compete with leadered mRNAs for initiating ribosomes in
vivo.
IF2 affects 70S ribosome binding to leaderless mRNAs.
Because of a recent report that IF2 protects the 23S rRNA of 70S ribosomes (11), toeprint assays were performed to investigate the effect of IF2 on 70S ribosome binding to leaderless mRNA. IF2 was present in a twofold molar excess over ribosomes and was prebound to ribosomes (37°C, 5 min) prior to addition of mRNA. Initiator tRNA was charged and formylated, as described previously (19). Addition of IF2 to 70S ribosomes reduced the toeprint signal observed for cI (Fig. 4A) and tetR (Fig. 4C) but had minimal effect on binding the gene V (Fig. 4B) leaderless mRNA. Because IF2 has a higher affinity for 30S subunits than for 70S ribosomes (18), the slight inhibition observed for IF2 in binding leaderless mRNA to 70S ribosomes might be minimal in vivo.
Concluding remarks.
The results reported here suggest that the conventional pathway
for translation initiation with leadered mRNAs (
6), involving
30S subunits bound by IFs, might not contribute significantly
to translation of leaderless mRNA. Given the in vivo competition
for 30S subunits by the multitude of SD sequence-containing
leadered mRNAs, it is reasonable to speculate that initiation
complexes of leaderless mRNA and 30S subunits might assemble
only through interaction with subunits lacking IF3. Based on
our in vitro ribosome binding assays, more
cI, gene
V, and
tetR leaderless mRNA was bound by 70S ribosomes than by 30S subunits,
consistent with an earlier report for
cI mRNA (
1). The higher
binding affinity, or increased stability, suggests that leaderless
mRNAs with AUG or GUG start codons might more readily form initiation
complexes with 70S ribosomes, thereby relieving competition
for 30S subunits; expression levels from leaderless mRNAs might
then reflect the abundance, or available pool, of free 70S ribosomes.
Translation initiation via 70S ribosomes would represent a novel
initiation pathway and might contain mRNA-ribosome interactions
not present in the conventional pathway that initiates with
30S subunits.

ACKNOWLEDGMENTS
We thank Richard Calendar for providing plasmid pGCV22; Uttam
RajBhandary for providing plasmids pQE16-MTF and pQE60-MetRS;
Claudio Gualerzi for providing plasmids encoding IF1, IF2, and
IF3 and protocols for IF purification; and Anton Vila for providing
helpful advice on tRNA charging reactions. Inita Forti is acknowledged
for inspiring discussions.
This work was supported by grant GM45923 from the National Institutes of Health and, in part, by funds received through the Research Challenge program of the Ohio Board of Regents. S.M.O. thanks the Miami University Graduate School for a Graduate Student Achievement Award.

FOOTNOTES
* Corresponding author. Mailing address: Department of Microbiology, Miami University, Oxford, OH 45056. Phone: (513) 529-1694. Fax: (513) 529-2431. E-mail:
janssegr{at}muohio.edu.

Present address: Department of Pediatric Infectious Disease, Vanderbilt University School of Medicine, Nashville, TN 37232. 

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Journal of Bacteriology, December 2002, p. 6730-6733, Vol. 184, No. 23
0021-9193/02/$04.00+0 DOI: 10.1128/JB.184.23.6730-6733.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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