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inhibition of rRNA synthesis, which is completely reversible when the hormone is withdrawn.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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