Glucocorticoids Selectively Inhibit Translation ...
Vol. 7, No. 8
MOLECULAR AND CELLULAR BIOLOGY, Aug. 1987, p. 2691-2699
0270-7306/87/082691-09$02.00/0
Copyright ? 1987, American Society for Microbiology
Glucocorticoids Selectively Inhibit Translation of Ribosomal Protein
mRNAs in P1798 Lymphosarcoma Cells
ODED MEYUHAS,1 E. AUBREY THOMPSON, JR.,2 AND ROBERT P. PERRY3*
Developmental Biochemistry Research Unit, Institute of Biochemistry, The Hebrew University-Hadassah Medical School,
Jerusalem, Israel'; Department of Biology, University of South Carolina, Columbia, South Carolina 292082; and Institute
for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania 191113
Received 26 February 1987/Accepted 27 April 1987
When P1798 murine lymphosarcoma cells are exposed to i0-7 M dexamethasone, there is a dramatic
inhibition of rRNA synthesis, which is completely reversible when the hormone is withdrawn. In the present
experiments we examined whether dexamethasone treatment causes any alteration in the accumulation or
utilization of mRNAs that encode ribosomal proteins (rp mRNAs). No effect on the accumulation of six different
rp mRNAs was detected. However, the translation of five of six rp mRNAs was selectively inhibited in the
presence of the hormone, as judged by a substantial decrease in ribosomal loading. Normal translation of rp
mRNA was resumed within a few hours after hormone withdrawal. In untreated or fully recovered cells, the
distribution of rp mRNAs between polyribosomes and free ribonucleoprotein is distinctly bimodal, suggesting
that rp mRNAs are subject to a particular form of translational control in which they are either translationally
inactive or fully loaded with ribosomes. A possible relationship between this mode of translational control and
the selective supression of rp mRNA translation by glucocorticoids is discussed.
rate of 18S and 28S rRNA synthesis is markedly reduced,
apparently because of an induced deficiency of a transcriptional initiation factor that is required for polymerase I
activity (6-8). We observed that the synthesis and turnover
of rp mRNAs are not significantly affected by the hormone
treatment but that the translation of most of these mRNAs,
as judged by their polysomal association, is selectively
inhibited in a reversible manner. This growth-arrested system, in which the syntheses of rRNA and rps are generally
coordinated, provided new insights into the translational
control mechanism that governs the synthesis of rps in
The biogenesis of eucaryotic ribosomes requires an equimolar accumulation of four RNAs and over 70 different
ribosomal protein (rp) molecules. This stoichiometry is
maintained over a wide range of growth rates and other
physiological conditions by coordinate regulation at various
levels of gene expression. For example, transcriptional
control has been reported for yeast rp genes after heat shock
(25) and nutritional upshift (13, 24). Control at the RNAprocessing level appears to operate for at least one rp in
yeasts (10, 57) and in Xenopus laevis (4). An increase in the
relative abundance of rp mRNAs in regenerating rat liver is
also the result of a posttranscriptional mechanism (16; D.
Peleg and 0. Meyuhas, unpublished data). Regulation of rp
mRNA translation has been observed in gene dosage compensation experiments in yeasts (19, 42, 57), during embryological development of Drosophila melanogaster (2, 23)
and X. laevis (3, 45, 46), in secretory glands of D. melanogaster (50), in insulin-treated chicken embryo fibroblasts (11,
20), and in growth-stimulated mouse fibroblasts (17). Finally,
a balanced accumulation of rps is also achieved by modulating their turnover, as observed in yeasts (1, 15, 57), mouse
oocytes (28), and differentiating rat myoblasts (21).
Although the syntheses of rRNA and rps are coordinated
in proliferating cells and after growth stimulation, they are
clearly not coordinated in many cases of growth arrest. For
example, in mammalian cells and mouse oocytes, when
rRNA synthesis is selectively inhibited by low concentrations of actinomycin D, rps continue to be synthesized at
normal rates (9, 28, 56). Also, when rat myoblasts are
induced to differentiate into myotubes, rRNA synthesis
decreases to 10 to 20% of the former rate, while the synthesis
of rps remains constant (21, 27).
The present experiments were designed to examine the
relationship between rRNA synthesis and rp production in
the glucocorticoid-responsive murine lymphosarcoma cell
line P1798 (54). In the presence of 10-7 M dexamethasone,
the proliferation of these cells is reversibly inhibited and the
*
mammalian cells.
MATERIALS AND METHODS
Cell culture. P1798.C7 cells were grown in suspension
under the conditions described by Cavanaugh and Thompson (7). Cells were exposed to 10-7 M dexamethasone for 24
h, centrifuged, suspended in fresh medium without dexamethasone, incubated at 37¡ãC for 45 min, recentrifuged,
suspended in fresh medium at a density of 2 x 105 to 5 x 105
cells per ml, and further incubated for 1.5 to 24 h.
Transcription in isolated nuclei. Isolation of nuclei from
P1798 cells, RNA elongation reactions, extraction of labeled
RNA, and hybridization with nitrocellulose strips bearing
immobilized DNA containing mouse rRNA sequences were
done as previously described (7). The autoradiograms were
scanned, integrated, and calibrated as described elsewhere
(8). Nonspecific autoradiographic signals were assessed by
using pBR322 DNA as a control.
Polysome fractionation. About 100 ml of P1798 cells (3 x
105 to 5 x 105 cells per ml) were harvested by pouring the
culture over frozen crushed phosphate-buffered saline (0.125
M NaCl, 10 mM NaH2PO4, 30 mM K2HPO4), washed once
with phosphate-buffered saline, and frozen at -70¡ãC. Cells
were thawed and suspended in 10 mM NaCl-10 mM Tris
hydrochloride (pH 7.4-S15 mM MgCl2 and lysed with 1.2%
Triton X-100-1.2% deoxycholate by brief mixing (3 s on a
Vortex mixer) before and after 3 min of incubation at 0¡ãC.
Corresponding author.
2691
2692
MEYUHAS ET AL.
Nuclei were pelleted by centrifugation for 2.5 min in a
microcentrifuge at 4¡ãC. The postnuclear supernatant was
diluted with an equal volume of 25 mM Tris hydrochloride
(pH 7.5)-10 mM MgCl2-25 mM NaCl-0.05% Triton
X-100-0.14 M sucrose-500 ,ug of heparin per ml. A 1-ml
portion of this suspension was layered over 35 ml of 15 to
45% (wt/wt) sucrose gradient with a 2-ml cushion of 45%
sucrose. The sucrose solutions contained 2 mM Tris hydrochloride (pH 7.5 at 4¡ãC), 25 mM NaCl, 5 mM MgCl2, and 100
,ug of heparin per ml. The gradients were centrifuged at 24
krpm for 285 min at 3¡ãC in a Beckman SW27 rotor. After
centrifugation, the A260 was monitored with a 2-mm flow cell
attached to a Gilford 2000 recording spectrophotometer, and
13 fractions of about 3 ml each were collected and adjusted
to 1% sodium dodecyl sulfate (SDS). RNA was extracted
from fractions 2 to 13 as previously described (34).
Total poly(A)+ RNA preparation and blot analysis. Total
RNA from P1798 was extracted by the hot-phenol method
essentially as described by Scherrer (47). Poly(A)+ RNA
was isolated as described previously (44). For dot-blot
analysis, RNAs were incubated with 7% formaldehyde-6x
SSC (lx SSC is 0.15 M NaCl plus 0.015 M sodium citrate) at
68¡ãC for 10 min and then diluted 7.5-fold with ice-cold lOx
SSC and applied to nitrocellulose or Nytran sheets
(Schleicher & Schuell, Inc., Keene, N.H.) with a dot-blot
manifold. RNA transfer (Northern) blot analysis was done
by the method of Thomas (53) with the exception that
Nytran was used instead of nitrocellulose. The blots were
baked under vacuum for 2 h at 80¡ãC, washed with 0.1 x
SSC-1% SDS at 68¡ãC for 1 h, and prehybridized for 3 to 6 h
at 420C with S x SSC-50% formamide-0.5% SDS-500 ,ug of
heparin per ml-0.1% sodium PPj-100 ,ug of salmon sperm
DNA per ml-50 mM sodium phosphate buffer, pH 7.0.
Hybridization was initiated by the addition of heatdenatured 32P-labeled probe and an additional 100 ,ug of
salmon sperm DNA per ml to the prehybridization mix.
Filters were incubated at 42¡ãC for 36 to 44 h and then washed
four times with 1 x SSC at room temperature (5 min each
time), washed twice with O.1lx SSC-0.25% SDS at 55¡ãC (30
min each time), and quickly rinsed with 0.1 x SSC.
Probes. rRNA probe p123 is a pBR322 recombinant containing a 3.2-kilobase (kb) SalI fragment of mouse rDNA
encoding approximately 3,000 bases of the promoterproximal end of mouse pre-rRNA (35). 5S RNA probe pTH1
is a plasmid containing the Syrian hamster 5S RNA gene
(18). rp intron probes used were p3A/13, a 0.92-kb PvuIIEcoRI fragment from intron 3 of the L32 gene, p3A, subcloned into pUC12 (14); pIVS3, a 0.56-kb HindIII fragment
from intron 3 of the L30 gene subcloned into pUC9 (58); and
pIVS2-S16, a 0.9-kb SmaI-BamHI fragment from intron 2 of
the S16 gene (55) subcloned into pUC18. rp recombinant
cDNA probes used were pL32, pL30, and pS16 cloned
cDNAs (34). Isolated fragment probes used were as follows:
a 0.97-kb fragment containing the L32 processed gene, p4A
(14), joined to the 5' and 3' flanks of p3A; a 0.51-kb
SacII-XbaI fragment containing an L30 processed gene,
derived from plcXba (58); 0.29- and 0.33-kb EcoRI-HindIII
fragments bearing the cDNA inserts of pS16 and L18,
respectively, derived from subclones in pUC8 (la) of the
original cloned cDNAs (34); a 1.1-kb EcoRI fragment containing a mouse L19 processed gene (A. Shiran and 0.
Meyuhas, unpublished data); a 1.2-kb MspI fragment containing L7 cDNA (34); a 1.15-kb PstI fragment containing
mouse actin cDNA (36); a 1.3-kb PstI fragment containing
mouse hypoxanthine-guanine phosphoribosyl transferase
(HGPRT) cDNA (26); a 1-kb PstI fragment containing the
MOL. CELL. BIOL.
r RNA
r RNA
z
E0
0L
rRNA
IL
5S
5S
5Ss
5S
rRNA
Control
6 hr Dex
24 hr Dex
24 hr Dbx
S hr Reue
FIG. 1. Relative transcription of rDNA and 5S DNA in dexamethasone (dex)-treated P1798 cells. Nuclei were isolated from 108
untreated cells (control) at various times after addition of dexamethasone to the culture (final concentration, 1o-7 M) or 6 h after its
withdrawal (rescue). [32P]RNA, labeled during a nuclear runon
assay, was hybridized with nitrocellulose strips containing dots of
plasmid DNA bearing rRNA or 5S RNA sequences (pI23 and pTH1,
respectively).
mouse
glyceraldehyde-3-phosphate dehydrogenase (GA3PD)
cDNA (kindly provided by P. Curtis); and a 0.62-kb PstI
fragment containing human superoxide dismutase-I (SOD)
cDNA (51), kindly provided by Y. Groner.
RESULTS
Glucocorticoids have a marked inhibitory effect on the
transcription of rRNA in lymphosarcoma P1798 cells (7). To
monitor the kinetics of the inhibition of rRNA transcription
and its recovery, we performed nuclear runon experiments
with nuclei isolated at selected time points after hormonal
treatment and withdrawal (Fig. 1). By 6 h after the addition
of dexamethasone, the rate of transcription of the 45S
precursor of 18S and 28S rRNA dropped to about 50% of the
rate in exponentially proliferating cells, and by 24 h it was
negligible (99% inhibition). Moreover, within 6 h after removal of the hormone, the rRNA transcription rate returned
to about 75% of the control level. In contrast, the transcription of SS RNA remained unaffected throughout the cycle of
hormonal treatment and withdrawal.
To determine whether the synthesis of rp mRNA is
coordinately regulated with that of rRNA under these conditions, we examined poly(A)+ RNA from normally proliferating, dexamethasone-treated, and recovering P1798 cells
by dot-blot analysis with a set of probes specific for rps L32,
L30, and S16 (Fig. 2). Intron probes, which are suitable for
selectively detecting rp nuclear transcripts, and cDNA
probes appropriate for detecting total rp mRNA sequences
were used. With both sets of probes the hybridization signals
were essentially the same for all three categories of cells,
indicating that the levels of both intron-containing nuclear
transcripts and cytoplasmic mRNA were unaffected by the
dexamethasone treatment. Owing to the generally rapid
turnover of nuclear transcripts (t412 < 30 min [5, 48]), a
glucocorticoid-mediated diminution in transcription rate
would be expected to cause a significant decrease in the
steady-state level of the nuclear transcripts 24 h after expo-
INHIBITION OF TRANSLATION OF RIBOSOMAL PROTEIN mRNA
VOL. 7, 1987
L32
L30
Intron
S16
probe
Ci'l.*f*-
9
D
C2
*
0
..;..
0
W
*
*
4
*
Exons
C1I
0
0
*
C2
W *
pg 1
probe
0
*
0
*
*
0
0
'*
*@
*
*
*
2
2
4
2
4
FIG. 2. Relative abundance of rp transcripts in P1798 cells. Total
poly(A)+ RNA was isolated from P1798 cells treated for 24 h with
10' M dexamethasone (D) or 24 h after hormone withdrawal (W)
and from parallel untreated cultures (Cl and C2, respectively). RNA
samples were dot blotted onto nitrocellulose sheets in single and
double amounts as indicated and initially hybridized with a set of
intron-specific probes derived from the L32, L30, and S16 genes
(intron probe). After melting off and decay of the initial signal, the
blots were rehybridized with the corresponding cDNA probes
(exons probe). The autoradiogram of the blot hybridized with the
intron probe was exposed approximately 20 times longer than that of
the exons blot. Wheat germ tRNA, blotted in similar quantities,
gave no detectable signal with these probes.
sure to the hormone. The fact that dexamethasone does not
alter the content of rp nuclear transcripts implies that the
transcription of the corresponding rp genes is not inhibited
by the hormone. Furthermore, the unchanged content of
mature rp mRNAs indicates that the turnover rate of rp
mRNAs is also unaffected by dexamethasone.
The failure of dexamethasone to induce changes in rp
mRNA abundance was verified by Northern blot analysis of
the L32, L30, and S16 mRNAs, as well as those encoding rps
L7, L18, and L19 (Fig. 3). In this experiment we also
determined the content of several other mRNA species
encoding proteins in the housekeeping category, viz., actin,
HGPRT, GA3PD, and SOD. The accumulation of all these
mRNAs, including those encoding the rps, was not detectably affected by the hormone treatment and thus was not
coupled to rRNA synthesis under these conditions.
To examine whether the translation of rp mRNAs is
affected by glucocorticoid treatment, we monitored their
relative loading onto polyribosomes. The presence of an
mRNA in polyribosomes is generally a reliable indicator of
its translational activity (32); its existence as free messenger
ribonucleoprotein (mRNP) unequivocally demonstrates its
translational inactivity. P1798 cells were grown in the presence of dexamethasone for 24 h, and then the hormone was
removed. At various times thereafter, cells were harvested
and their polysomes were size fractionated by sucrose
gradient centrifugation. In dexamethasone-treated cells approximately 42% of the ribosomes were engaged in polysomes, as judged by the proportion of total ribosomal
particles, including subunits, that sedimented in the polysome region of the gradient (Fig. 4a). The proportion was
2693
similar in cells harvested 1.5 h after hormone withdrawal
(data not shown), but by 3 h the polysome fraction increased
to 66% (Fig. 4b) and remained essentially unchanged thereafter. The polysomal profile of cells harvested 24 h after
hormonal withdrawal (Fig. 4c) was indistinguishable from
that of untreated cells (data not shown). The polyribosomal
association of the various rp mRNAs and of the mRNAs
encoding other housekeeping proteins was determined by
dot-blot analysis of each gradient fraction. Figure 5 shows
examples of this analysis for the L32, L18, actin, and SOD
mRNAs in dexamethasone-treated cells and at various times
after hormonal withdrawal. A striking difference in the
polysomal distribution of rp mRNAs and non-rp mRNAs is
apparent. This difference is even better appreciated by a
quantitative analysis of these similar data (Fig. 6-8). Since,
in some cases, the rp and non-rp probes were sequentially
hybridized to the same sets of filters, the observed differences in polysome distribution cannot be due to experimental variability but rather must reflect the distinctive behavior
of these two classes of mRNA.
The polysomal distribution of two representative mRNAs,
L32 and actin, was observed to be essentially the same in
untreated cells as in cells harvested 24 h after hormone
withdrawal (Fig. 8). Thus, the profiles of the 24-h withdrawal
samples are considered to be typical of normal exponentially
growing cells. Maximum loading of the mRNAs into polysomes was generally achieved within 3 h after withdrawal of
dexamethasone and in all cases within 18 h (Fig. 7). These
I
I
2
3 1 4 | 5 I B
1
WOW oW oWo W oWo
1
7 1 8
9
10
W DWD W OWO
28S
MS2
23S
18S
16S
as~_
-n
so-
So
4
FIG. 3. Northern blot analysis of poly(A)+ RNA from P1798
cells. A 3-,ug sample of total poly(A)+ RNA from cells harvested 24
h after dexamethasone treatment (D) or 24 h after hormone withdrawal (W) was size fractionated in alternating lanes by electrophoresis in a 1% agarose gel containing 2.2 M formaldehyde. After
transfer of RNA onto a Nytran sheet, each pair of lanes (W and D)
was hybridized with one of the following isolated fragment probes
(for details see Materials and Methods): 1, L32; 2, GA3PD; 3, L18;
4, actin; 5, SOD; 6, L7; 7, L30; 8, HGPRT; 9, S16; 10, L19.
Autoradiographic exposures were varied to give comparable intensities for the different mRNA species. The locations of rRNA and
bacteriophage RNA size markers (37) are indicated.
2694
MEYUHAS ET AL.
MOL. CELL. BIOL.
(0
N
.2
w
z
.
POLYSOMES-
POLYSOMES-1
o
0
P9
o3
5 7a 9
.15
12
C
0
3
4
3
5
7
9
II89101
M 60 40
5
E
C
0
(0
N
!7%
~~~~~~~POLYSOMESI
w
z
0
2
3
4 5 6 7 8 9 10 II
FRACTION NUMBER
FIG. 4. Absorbance profiles of polysomes and subpolysomal particles fractionated on sucrose gradients. A postnuclear supernatant from
about 3.5 x iO1 P1798 cells was centrifuged through a 35-ml 15 to 45% sucrose gradient. A260 was continuously monitored, and fractions were
collected from bottom to top of the gradient. Polysomal profiles are from cells treated for 24 h with dexamethasone (a), cells harvested 3 h
after hormone withdrawal (b), or cells harvested 24 h after hormone withdrawal (c). M, Monosomes; 40, 40S subunits; 60, 60S subunits. The
number of ribosomes associated with mRNA in individual polysomal peaks is indicated in panel c.
findings are consistent with previous measurements of other
indices of cell growth and proliferation (7).
As seen in Fig. 6 to 8 and summarized in Table 1, the
ribosome loading of rp mRNAs differs from that of the
non-rp mRNAs in two important respects: sensitivity to
glucocorticoid treatment and overall efficiency under normal
circumstances. In dexamethasone-treated cells, a high proportion of the rp mRNA (with the exception of L7) occurred
mainly as free mRNP, indicating that it is in a translationally
inactive state under these conditions. The proportion of
these rp mRNAs that was polysome associated in hormonetreated cells ranged from 22 to 38%, compared with 59 to
76% in untreated and recovered cells (Table 1). In contrast,
the translation of the non-rp mRNAs was much less affected
by the hormone treatment. These mRNAs, which are 86 to
93% polysome associated under normal conditions, were
still 56 to 86% polysome associated after 24 h of dexamethasone treatment. L7 mRNA was exceptional among the rp
mRNAs examined in that its translation was only moderately
affected by the hormone treatment. These striking differ-
ences in ribosome loading, which were also observed in
another independent experiment, are apparently not attributable to differences in mRNA size or codon number since
these parameters clearly overlap among the rp mRNA and
non-rp mRNA clases (Fig. 3; Table 1).
In the untreated and recovered cells, the profiles of the rp
mRNAs were distinctly bimodal, suggesting two discrete
populations of mRNA molecules, distinguished by being
either translationally active (polysome associated) or translationally inactive (free mRNP). Interestingly, the translationally active population is fully loaded with ribosomes, as
indicated by the estimated spacing of about 31 codons per
ribosome in the peak polysome fractions (Table 1). This
spacing, which is also characteristic of the non-rp mRNAs
that were examined, was considered to be close to the
theoretical maximum (22). Thus, in normal growing cells, the
rp mRNAs appear to exhibit an all-or-none behavior, alternating between a translationally inactive state and an active
state in which they are translated at near maximum effi-
ciency.
INHIBITION OF TRANSLATION OF RIBOSOMAL PROTEIN mRNA
VOL. 7, 1987
WITHDRAWAL
TIME (hrs)
0
'*
*
000-*
z
*@@
*@
-L
L32
0.0*
*-06
0*@ @I.
90
-
u
2e*
ACTIN
00*
2695
* @0*
*0
0
1.5
3
6
9
12
18
24
*9*
0
1
0 *X
it
** *
..
+
*
. =__________1.
0
1.5
3
6
9
12
18
24
L18
*
0
7
.. =.
^0
0* 0~~
SOD
* 6
8
I
2
4
6
8
10
2
12
FRACTION NUMBER
4
10
6
8
FRACTION NUMBER
12
FIG. 5. Dot-blot analysis of the distribution of various mRNAs in polysomal and subpolysomal fractions. Postnuclear supernatants from
24-h dexamethasone-treated cells (0 withdrawal time), as well as at various times after hormone withdrawal, were fractionated through
sucrose gradients (see Fig. 4 for the respective polysomal profiles of selected withdrawal times). RNA was extracted from each fraction, and
aliquots representing equal volumes were dot blotted onto Nytran sheets. Blots were initially hybridized with 32P-labeled isolated fragment
probes for rps L32 and L18 (upper two panels). After melting off and decay of the rp signals, the blots were rehybridized with actin and SOD
probes, respectively (lower two panels). The vertical line separates the polysomal fractions (left) and subpolysomal fractions (right). The lack
of uniformity among the different gradients with respect to this partitioning results from slight variations in centrifugation conditions and
volume of fractions. Owing to variations in the amounts of cell extract applied to each gradient, quantitative comparisons between individual
fractions can only be made horizontally within the same gradient.
DISCUSSION
The results of these studies demonstrate that the relative
abundance of rp mRNA in P1798 lymphosarcoma cells
remains unchanged when rRNA synthesis is almost com-
pletely suppressed by glucocorticoid treatment. Neither the
synthesis nor the turnover of rp mRNAs was detectably
affected under these conditions. Yet, despite the continued
accumulation of rp mRNAs, there was, in fact, a reduction in
their usage as indicated by the selective decrease in their
TABLE 1. Comparison of the polysomal association of various mRNAs in dexamethasone-treated and recovered P1798 cells
Protein
mRNA size'
(kb)
No. of
codonsb
L32
L30
S16
L18
L19
L7
SOD
HGPRT
GA3PD
Actin
0.63
0.59
0.67
0.8
0.9
1.05
0.83
1.25
1.4
1.65
135
115
145
200
196
258
153
214
333
374
% mRNA associated with polysomes'
24-h
24-h
withdrawal
dexamethasone
22
22
26
21
38
51
76
56
68
86
59
59
69
68
76
72
93
87
86
90
Avg no. of
No. of codons
fibosomes
per ribosome'
per mRNAp
30
4-5
3-5
29
30
4-6
5-7
33
5-7
33
32
7-9
31
4-6
31
6-8
>10
>10
mRNA size derived from references 14, 34, 55. and 58 and from Fig. 3.
b Number of codons was derived from available amino acid sequences as follows: mouse L32 (14). mouse L30 (58). mouse S16 (55). rat L19 (8a), rat L7 (A. Lin,
Y.-L. Chan, J. McNally, D. Peleg, 0. Meyuhas, and I. G. Wool, unpublished data), human SOD (51), mouse HGPRT (33), chicken GA3PD (40), rat actin (39). The
value for L18 was estimated from protein molecular weight (59). In those cases in which the sequence is not of mouse origin we assumed that the number of codons
in the mouse counterpart is approximately the same. This assumption seems justified in view of the strong cross-hybridization of nucleotide sequences (Fig. 3),
which indicates a high degree of evolutionary conservation.
C Data derived from Fig. 7.
d Values derived from a comparison of the peak polysomal fractions in cells harvested 24 h after hormone withdrawal (Fig. 6) and the corresponding polysomal
profile (Fig. 4c).
e Values obtained by dividing the number of codons in an mRNA by the average number of ribosomes in the peak polysomal fraction.
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