Dermatomyositis and peritoneal papillary serous carcinoma

We describe an unusual case of peritoneal papillary serous carcinoma (PPSC) arising in a female patient with dermatomyositis (DM). Despite periodic extensive searches for an underlying malignancy, no malignancy had been detected in this patient during the first 2.5 years after the diagnosis of DM. It was only when the patient presented with pleural effusion and ascites that the underlying intra-abdominal malignancy was detected by laparoscopy. Treatment with four cycles of pre-operative chemotherapy (taxol and cisplatin) resulted in tumor regression with amelioration in the muscular manifestation of the DM, but without parallelic amelioration in the skin manifestations of the DM. Explorative laparotomy confirmed the presence of papillary serous carcinoma in the omentum, surface of the left ovary and the retroperitoneal lymph nodes, and established the diagnosis of PPSC. Following two cycles of postoperative chemotherapy, the patient is alive with no evidence of internal malignancy. However, although muscle strength and enzymes have remained normal, no effect on the skin manifestation of DM has been observed. This case illustrates that, alongside the more frequently occurring ovarian carcinoma, PPSC should also be considered in the differential diagnosis of the underlying malignancy that may occur in the female patient with DM.

Recognition of the universally conserved 3'-CCA end of tRNA by elongation factor EF-Tu

Escherichia coli tRNA(Val) with pyrimidine substitutions for the universally conserved 3'-terminal adenine can be readily aminoacylated. It cannot, however, transfer valine into polypeptides. Conversely, despite being a poor substrate for valyl-tRNA synthetase, tRNA(Val) with a 3'-terminal guanine is active in in vitro polypeptide synthesis. To better understand the function of the 3'-CCA sequence of tRNA in protein synthesis, the effects of systematically varying all three bases on formation of the Val-tRNA(Val):EF-Tu:GTP ternary complex were investigated. Substitutions at C74 and C75 have no significant effect, but replacing A76 with pyrimidines decreases the affinity of valyl-tRNA(Val) for EF-Tu:GTP, thus explaining the inability of these tRNA(Val) variants to function in polypeptide synthesis. Valyl-tRNA(Val) terminating in 3'-guanine is readily recognized by EF-TU:GTP. Dissociation constants of the EF-Tu:GTP ternary complexes with valine tRNAs having nucleotide substitutions at the 3' end increase in the order adenine < guanine < uracil; EF-Tu has very little affinity for tRNA terminating in 3' cytosine. Similar observations were made in studies of the interaction of 3' end mutants of E. coli tRNA(Ala) and tRNA(Phe) with EF-Tu:GTP. These results indicate that EF-Tu:GTP preferentially recognizes purines and discriminates against pyrimidines, especially cytosine, at the 3' end of aminoacyl-tRNAs.

Antibodies to mycoplasma fermentans in HIV-positive heterosexual patients: seroprevalence and association with AIDS

There are conflicting reports concerning the prevalence of Mycoplasma fermentans in HIV-positive patients and its association with AIDS. Serum antibodies to M. fermentans were measured by a modified immunoblotting technique in 48 HIV-positive heterosexual patients and in 30 HIV-negative heterosexual controls. Antibodies to M. fermentans were detected in 19 (40%) of HIV-positive patients and in three (10%) of the HIV-negative controls (P = 0.01). The prevalence of antibodies to Mycoplasma hominis and to Ureaplasma urealyticum was similar in both groups. In the HIV-positive group, 16/19 (84%) M. fermentans-positive patients developed AIDS, compared to eight of 29 (28%) M. fermentans-negative patients (P = 0.0004). The HIV-positive patients with antibodies to M. fermentans had a lower CD4+ cell count and a higher prevalence of antibodies to the other mycoplasma tested (P = 0.007 and P = 0.03, respectively), as compared to the patients without antibodies to M. fermentans. These findings may suggest that the presence of antibodies to M. fermentans indicate an opportunistic infection. Of the 19 M. fermentans-positive patients, 11 were positive on the first examination, and eight became positive during the follow-up period. Seven out of these eight patients developed antibodies to M. fermentans before the development of AIDS. Therefore, the possibility exists that M. fermentans might influence the development of AIDS.

Role of acceptor stem conformation in tRNAVal recognition by its cognate synthetase

Although the anticodon is the primary element in Escherichia coli tRNAValfor recognition by valyl-tRNA synthetase (ValRS), nucleotides in the acceptor stem and other parts of the tRNA modulate recognition. Study of the steady state aminoacylation kinetics of acceptor stem mutants of E.coli tRNAValdemonstrates that replacing any base pair in the acceptor helix with another Watson-Crick base pair has little effect on aminoacylation efficiency. The absence of essential recognition nucleotides in the acceptor helix was confirmed by converting E.coli tRNAAlaand yeast tRNAPhe, whose acceptor stem sequences differ significantly from that of tRNAVal, to efficient valine acceptors. This transformation requires, in addition to a valine anticodon, replacement of the G:U base pair in the acceptor stem of these tRNAs. Mutational analysis of tRNAValverifies that G:U base pairs in the acceptor helix act as negative determinants of synthetase recognition. Insertion of G:U in place of the conserved U4:A69 in tRNAValreduces the efficiency of aminoacylation, due largely to an increase in K m. A smaller but significant decline in aminoacylation efficiency occurs when G:U is located at position 3:70; lesser effects are observed for G:U at other positions in the acceptor helix. The negative effects of G:U base pairs are strongly correlated with changes in helix structure in the vicinity of position 4:69 as monitored by19F NMR spectroscopy of 5-fluorouracil-substituted tRNAVal. This suggests that maintaining regular A-type RNA helix geometry in the acceptor stem is important for proper recognition of tRNAValby valyl-tRNA synthetase.19F NMR also shows that formation of the tRNAVal-valyl-tRNA synthetase complex does not disrupt the first base pair in the acceptor stem, a result different from that reported for the tRNAGln-glutaminyl-tRNA synthetase complex.

Septic arthritis due to bacteroides fragilis after pilonidal sinus resection in a patient with rheumatoid arthritis

Bacteroides fragilis is a rare cause of septic arthritis. Most patients with B. fragilis septic arthritis have a chronic joint disease, particularly rheumatoid arthritis, and sources of infection are lesions of the gastrointestinal tract and the skin. We report a 69-year-old male, who developed B. fragilis septic arthritis after pilonidal sinus resection. High level of suspicion of development B. fragilis septic arthritis must be present in patients with chronic joint disease in whom gastrointestinal or skin surgery was previously performed.

Localization of the major ethidium bromide binding site on tRNA

Binding of ethidium bromide to Escherichia coli tRNAVal and an RNA minihelix based on the acceptor stem and T-arm of tRNAVal was investigated by 19F and 1H NMR spectroscopy of RNAs labeled with fluorine by incorporation of 5-fluorouracil. Ethidium bromide selectively intercalates into the acceptor stem of the tRNAVal. More than one ethidium bromide binding site is found in the acceptor stem, the strongest between base pairs A6:U67 and U7:A66. 19F and 1H spectra of the 5-fluorouracil-substituted minihelix RNA indicate that the molecule exists in solution as a 12 base-paired stem and a single-stranded loop. Ethidium bromide no longer intercalates between base pairs corresponding to the tRNAVal acceptor stem in this molecule. Instead, it intercalates between base pairs at the bottom of the long stem-loop structure. These observations suggest that ethidium bromide has a preferred intercalation site close to the base of an RNA helical stem.

Hyperreactive malarial splenomegaly and AIDS: proguanil prevents recurrent pneumonia

An Ethiopian immigrated to Israel and gave a positive test result for human immunodeficiency virus. Soon after, he was thrice admitted to hospital in 3 months for severe pneumonia. Acquired immunodeficiency syndrome was diagnosed, as was hyperreactive malarial splenomegaly (HMS). Thereafter, during maintained proguanil treatment for HMS, the patient remained well for 16 months. Then, after another pneumonic illness, pneumococcal vaccine was administered. Proguanil was maintained for another 10 months and, despite declining CD4 cell counts, good health continued for a further 18 months.

Postoperative complications after splenectomy for hematologic malignancies

OBJECTIVE

The authors analyzed the frequency and character of postoperative complications after splenectomy in patients with hematologic malignancies, and correlated these findings with preoperative conditions that could have predicted their outcome.

SUMMARY BACKGROUND DATA

Splenectomy is performed for hematologic malignancies for diagnostic and therapeutic indications. The role of splenectomy for lymphoproliferative and myeloproliferative malignancies is complex and sometimes controversial.

METHODS

The medical records of 135 patients undergoing splenectomies for hematologic malignancies at Roswell Park Cancer Institute from January 1, 1984 to December 31, 1993 were reviewed retrospectively. These included Hodgkin's disease (HD), hairy cell leukemia (HCL), non-Hodgkin's lymphoma (NHL), chronic lymphocytic leukemia (CLL), chronic myelogenous leukemia (CML), and a miscellaneous group.

RESULTS

The overall postoperative complication and mortality rates for all patients were 52% and 9%, respectively. The complication rate was 63% for patients whose spleens weighed greater than 2000 g, and 29% for patients whose spleens weighed less than 2000 g (p = 0.001). Seventy-three percent of the postoperative deaths were due to septic complications, only one of which was caused by an encapsulated organism. Complications occurred in less than 20% of patients with the diagnosis of HD and HCL; more than 50% of patients with NHL, CLL, and CML suffered postoperative complications.

CONCLUSIONS

Splenectomy performed in patients with hematologic malignancies is a potentially morbid procedure. Splenic size was the only preoperative factor found to be predictive of postoperative complications. The complication rate differed significantly between the different diagnostic subgroups.

The in vivo bioactivity of vascular endothelial growth factor/vascular permeability factor is independent of N-linked glycosylation

The carbohydrate moieties of glycoprotein hormones or growth factor molecules may have a variety of effects that impact biological potency. Vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF), is a 45 kD heparin-binding, endothelial cell (EC) specific mitogen with a putative N-linked glycosylation site. Recent studies have shown that VEGF/VPF may successfully augment collateral development in animal models of myocardial and hindlimb ischemia. The extent to which glycosylation of the 75 asparagine site affects the angiogenic properties of VEGF/VPF has not been studied in vivo. Specifically unaddressed to date is the concern that nonglycosylated VEGF/VPF may be less stable, and therefore characterized by a shorter half-life, reducing its utility for therapeutic angiogenesis. Accordingly, the purpose of this study was to investigate the extent to which posttranslational modification, specifically glycosylation, mofies the angiogenic properties of VEGF/VPF in vivo. Glycosylated (g+) recombinant human VEGF165 was purified from media conditioned by Chinese hamster ovary (CHO) cells. Nonglycosylated (g-) VEGF165 was expressed, purified and refolded from E. coli. The purity of both materials was assessed by silver-stained SDS/PAGE and characterized by the presence of a single amino terminal sequence as indicated by Edman degradation. Tryptic mapping by reverse-phase HPLC confirmed that the potential glycosylation site at 75 asparagine was occupied by N-linked carbohydrate for the Chinese hamster ovary-derived VEGF/VPF, but not for E. coli-derived VEGF/VPF. The mitogenic effects of Chinese hamster ovary-derived (g+) VEGF165 and E. coli-derived (g-) VEGF165 wre studied in vitro using microvascular EC. At concentrations of VEGF/VPF ranging from 10(-4) to 10(2) nM, both produced similar concentration-dependent effects on EC proliferation. For in vivo studies, (g-) (n = 8) and (g+) (n = 8) formulations of VEGF/VPF were administered to New Zealand white rabbits with unilateral hindlimb ischemia. For (g-) versus (g+) VEGF/VPF-treated groups, respectively, calf blood pressure ratio was 0.40 +/- 0.04 versus 0.37 +/- 0.04; angiographic score (of collateral vessels) was 0.37 +/- 0.04 versus 0.35 +/- 0.04; capillary density (capillaries/mm2) at necropsy was 246.9 +/- 21.5 versus 253.9 +/- 18.8; and tissue perfusion (colored microspheres) was 92.8 +/- 5.5 versus 90.30 +/- 13.47 (all p = ns). Moreover, intravascular Doppler-based analyses of resting, maximum, and endothelium-dependent flow was similar for (g-) and (g+) VEGF/VPF. These in vitro and in vivo findings establish that the potential for VEGF/VPF to stimulate therapeutic angiogenesis persists unaltered in the nonglycosylated state.

A case report. Ectopic molars that cause osteomeatal complex obstruction

During a routine dental examination, a general dentist noticed facial asymmetry in a 42-year-old man. Dental radiography suggested a pathological condition in the sinuses. The dentist referred the patient to a head and neck surgeon for further examination. Computed tomography demonstrated ectopic molars in the maxillary sinuses blocking drainage of the sinuses.

Perspectives of practicing nurses on ethical issues in health care economics

Perspectives of nurses working in seven hospitals across the United States are described. Most nurses believed that all people should have equal access and quality of health care. However, most nurses were unwilling to pay more taxes or donate time to achieve these goals.

Infection of the amniotic cavity with Ureaplasma urealyticum in the midtrimester of pregnancy

The purpose of this study was to determine the clinical significance of invasion of the amniotic fluid (AF) with Ureaplasma urealyticum in the midtrimester of pregnancy. Amniotic fluid and cervical swabs obtained from 214 asymptomatic women in the midtrimester of pregnancy (16-20 weeks) were cultured for U urealyticum and Mycoplasma hominis. Inoculum size was determined by quantitative culture. Six of the 214 women (2.8%) had Ureaplasma in high titers in the AF. The pregnancy outcomes of 129 women were determined. Adverse pregnancy outcome occurred more frequently in women with a positive AF culture than in women with a negative AF culture (3/6 [50%] vs. 15/123 [12%], respectively; P = .035). We conclude that infection of the amniotic cavity with U urealyticum can be present in asymptomatic patients in the midtrimester of pregnancy and is a significant risk factor for spontaneous preterm labor and delivery. These observations suggest that routine culture of AF at the time of midtrimester amniocentesis can identify the group of patients at risk for a poor pregnancy outcome.

An institutional review of sarcomas of the large and small intestine

BACKGROUND

This study was done to review the institutional experience with the treatment of sarcoma of the small or large intestine.

STUDY DESIGN

Thirty-nine patients admitted between the years 1959 and 1987, with a diagnosis of sarcoma of the small or large bowel form the basis of this review.

RESULTS

At referral, 74 percent of the patients presented with peritoneal sarcomatosis. Only six patients underwent complete resection. The overall five-year survival rate was 20 percent. Patients with low grade tumors had median and five-year survival rates of 33.3 months and 44 percent, respectively, while patients with high grade tumors had median and five-year survival rates of 22.4 months and zero percent, respectively, p = 0.01. Patients undergoing complete resection had a median survival period of 33.3 months, while patients receiving less than complete resection had a median survival period of 15.4 months, p = 0.003. Factors found to be significant by multifactorial analysis included tumor size, grade, stage at presentation, and invasion of adjacent organs.

CONCLUSIONS

Sarcoma of the small and large bowel is an uncommon entity. Survival rates are relatively poor. Aggressive surgical intervention is the mainstay of therapy.

Antibodies to Ureaplasma urealyticum in women with intraamniotic infection and adverse pregnancy outcome

OBJECTIVE

To determine the association between antibody response to Ureaplasma urealyticum and pregnancy outcome in women with ureaplasmal invasion of the amniotic cavity.

METHODS

A cross section study was performed in our high risk pregnancy unit. Three groups of pregnant women were identified: 1) 271 women at the midtrimester of pregnancy (16-20 weeks), who underwent transabdominal amniocentesis for genetic indications 2) 161 consecutive women admitted with preterm labor and intact membranes. 3) 118 consecutive patients with preterm premature rupture of membranes. Amniotic fluids were cultured for Ureaplasma urealyticum, Mycoplasma hominis and also for other aerobic and anaerobic bacteria. Serum antibodies to Uu were measured by a specific enzyme-linked-immunosorbent-assay.

RESULTS

The prevalence of positive AF cultures with Uu in the 3 study groups was 2.9%, 4.3% and 17.8%, and the prevalence of antibodies to Uu in the AF-colonized patients was 50%, 86% and 57%, respectively. Adverse pregnancy outcome (preterm birth, low birth weight or fetal death) among all AF colonized women, was significantly higher in patients with antibodies to Uu than in those without antibodies; preterm delivery 90% vs. 43%, and low birth weight infants or fetal death 85% vs. 28%, p = 0.006 and 0.001 respectively.

CONCLUSION

Women with intraamniotic infection with Uu and elevated levels of antibodies to Uu, had a higher rate of preterm delivery, and low birthweight infants/fetal death than those without antibody to Uu.

Ureaplasma urealyticum cervical colonization as a marker for pregnancy complications

OBJECTIVES

To determine the clinical significance of cervical colonization with (Ureaplasma urealyticum (Uu) and its possible relationship to pregnancy outcome.

METHODS

Cervical cultures for Uu and serum antibodies to Uu were determined in four groups of pregnant women: (1) 117 women who underwent mid-trimester amniocentesis; (2) 47 women with preterm labor and intact membranes; (3) 34 women with preterm premature rupture of membranes; and (4) a control group of 315 healthy women with normal pregnancies. Statistical methods used were the chi-square and Fisher's exact tests.

RESULTS

A significant increase in the cervical colonization rate with Uu was detected in all study groups (62%, 77% and 74%, respectively) when compared with the control group (42%). Women at mid-trimester of pregnancy with a positive cervical culture and high levels of antibodies, had a higher rate of pregnancy complications than those with a negative culture and absence of antibodies (62 vs. 28%, respectively; P = 0.0006).

CONCLUSION

Cervical colonization with Uu when associated with elevated titers of antibodies to Uu, may serve as a marker for the identification of a subpopulation of women who are at high risk for the development of pregnancy complications.

Mycoplasma genitalium in the joints of two patients with arthritis

Mycoplasma genitalium was sought in synovial fluids from 13 patients, of whom five had Reiter's syndrome, four had rheumatoid arthritis, and one each had systemic lupus erythematosus, psoriatic arthritis, rheumatic fever and undefined arthritis. The mycoplasma was detected by a PCR assay in the knee joint of a 25-year-old man with Reiter's syndrome, from whom urethral ureaplasmas were isolated and whose synovial fluid mononuclear cells responded to ureaplasmal antigens in a proliferation assay. Mycoplasma genitalium was also detected in the knee joint during an exacerbation of arthritis in a 58-year-old man who had had seronegative juvenile polyarthritis that had evolved to seronegative rheumatoid arthritis.

Functional transfer RNAs with modifications in the 3'-CCA end: differential effects on aminoacylation and polypeptide synthesis

The trinucleotide CCA sequence is present at the 3' terminus of all mature tRNAs. Despite this high degree of conservation, we have been able to prepare in vitro transcripts of Escherichia coli tRNA(Val) with altered 3' termini that are readily aminoacylated and can function in polypeptide synthesis. Replacement of the 3'-terminal adenosine with either cytidine or uridine yields a tRNA(Val) variant that retains almost full aminoacylation activity, having specificity constants (Vmax/Km) 40-50% that of wild-type tRNA(Val). The tRNA(Val) variant with a 3'-terminal guanosine remains fully chargeable but is a poor substrate for valyl-tRNA synthetase, largely as the result of a decrease in the catalytic constant. End-group analysis revealed the absence of adenosine at the 3' end of the tRNA(Val) mutants and identified the nucleotide expected from the sequence of the DNA template as the predominant 3'-terminal residue; Val-cytidine was isolated from the aminoacylated C76 mutant. Val-tRNA(Val) with 3'-CCG is active in poly(U,G)-directed (Val, Phe) copolypeptide synthesis, whereas the tRNA(Val) mutants terminating in 3'-CCC and 3'-CCU, which are readily aminoacylated, are inactive. The differential effects of nucleotide substitution on aminoacylation and polypeptide synthesis suggest that the universally conserved 3'-CCA end of tRNAs is monitored at two or more steps in protein synthesis that have different nucleotide recognition specificities.

Nucleoside modifications stabilize Mg2+ binding in Escherichia coli tRNA(Val): an imino proton NMR investigation

The structures of in vitro transcribed Escherichia coli tRNA(Val), which lacks base modifications, and the native tRNA, which contains them, are very similar in the presence of excess Mg2+ (Kintanar, Yue, and Horowitz, unpublished results). To further probe the effects of base modifications on the structure of tRNA, the Mg2+ ion dependence of the downfield region of the 1H NMR spectrum of in vitro transcribed E. coli tRNA(Val) in aqueous phosphate buffer was investigated. The spectra indicate a remarkable conformational change in unmodified E. coli tRNA(Val) coincident with binding or release of Mg2+. Assignment of the imino proton resonances in the low Mg2+ form of the tRNA transcript allows a detailed description of the conformational change. There is near total disruption of the D stem and tertiary interactions in the absence of bound Mg2+. A new strong interaction between the U67-A6 base pair and the G50-U64 wobble pair is observed, indicating a substantial structural rearrangement at the junction of the acceptor and T stems. The binding constants of the strong Mg2+ binding sites in the D loop and near the D stem in unmodified tRNA(Val) are at least 2 orders of magnitude less than in tRNAVal containing base modifications. The metal ion binding site in the anticodon loop is somewhat stronger than metal ion binding sites in the D loop and stem in unmodified tRNA(Val), but it is still weaker than all strong Mg2+ binding sites in native tRNA(Val). Thus, one role of the base modifications found in tRNA is to stabilize or strengthen the Mg2+ binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Ureaplasma urealyticum in Reiter's syndrome

OBJECTIVE

To evaluate the role of Ureaplasma urealyticum (Uu) in the pathogenesis of Reiter's syndrome (RS).

METHODS

Infection with Uu was determined in 31 patients with RS and 28 patients with other arthritides by urethral, cervical and synovial fluid (SF) culture and by measuring anti-Uu serum antibody. Infection with Chlamydia trachomatis was determined by examining SF by a direct immunofluorescence technique, by a polymerase chain reaction and by measuring anti-C. trachomatis serum antibody. The proliferative response of SF and peripheral blood mononuclear cells (PBMC) to Uu antigens in patients with RS was compared to that of a control group. The effect that treatment of 6 patients with RS with ciprofloxacin had on repeated cultures, on titer of anti-Uu antibody and on mononuclear cell reactivity was measured sequentially.

RESULTS

The colonization rate of Uu in patients with RS (74%) was significantly greater than in patients with other arthritides (14%). Genital C. trachomatis isolation and serum anti-C. trachomatis antibody were uncommon in both groups (11 and 13%, respectively). SF mononuclear cells of the patients with RS proliferated specifically in response to Uu antigens [up to 6.9 stimulation index (SI)], as did their PBMC (up to 14.5 SI). In some patients, high anti-Uu antibody titers were measured in the serum. Clinical remission was observed in 4 of 6 patients and correlated with eradication of Uu, decrease in antibody titers and disappearance of mononuclear cell reactivity to Uu antigens.

CONCLUSION

Our findings suggest that Uu might be a causative agent or a trigger in the development of sexually acquired RS.

Effect of nucleoside modifications on the structure and thermal stability of Escherichia coli valine tRNA

Transfer RNA transcribed in vitro lacks the base modifications found in native tRNA. To understand the effect of base modifications on the structure of tRNA, the downfield region of the 1H NMR spectrum of in vitro transcribed E coli tRNAVal in aqueous phosphate buffer in the presence of excess Mg2+ was investigated. The resonances of all imino protons involved in hydrogen bonds in the helical stem regions and in tertiary interactions were assigned using two-dimensional nuclear Overhauser enhancement spectroscopy (NOESY) and one-dimensional difference nuclear Overhauser effect (NOE) methods. In addition, some aromatic C2 and C8 proton resonances as well as one amino proton resonance were assigned. The chemical shifts of the assigned resonances of unmodified E coli tRNAVal were compared with those of the native tRNA molecule under similar solution conditions. The similarity of the NMR data for unmodified and modified tRNA indicates that the in vitro transcribed tRNA has nearly the same solution structure as the native molecule in the presence of excess Mg2+. The only significant differences were the chemical shifts of resonances corresponding to protons in (or interacting with) bases, indicating the possibility of local structural perturbations. The thermal stability of E coli modified and unmodified tRNAVal in the presence of Mg2+ was also investigated by analyzing the temperature dependence of the imino proton spectra. Several tertiary interactions involving modified nucleosides in native E coli tRNAVal are less stable in the absence of base modifications.

Probing structural differences between native and in vitro transcribed Escherichia coli valine transfer RNA: evidence for stable base modification-dependent conformers

Structural differences between native (modified) and in vitro transcribed (unmodified) Escherichia coli tRNA(Val) were explored by comparing their temperature-absorbance profiles as a function of magnesium ion concentration and by probing their solution conformation with single- and double-strand-specific endonucleases. In vitro transcribed tRNA(Val) has a less ordered structure as monitored by thermal melting profiles; its Tm is appreciably lower than that of native tRNA(Val) at all Mg2+ concentrations. Structure probing experiments with nuclease S1 and ribonuclease V1 show that the unmodified tRNA(Val) transcript is more susceptible to nuclease attack at low Mg2+ concentrations, particularly in the D- and T-loops, indicative of at least a partial disruption of D-loop/T-loop interactions. These experiments also provide evidence for temperature-dependent alternative conformations of the anticodon loop of native tRNA(Val). Modified nucleosides are essential for the stability of these conformers; they cannot be detected in the unmodified in vitro transcript. The observations suggest that post-transcriptional modifications in tRNA allow the adoption of unique conformations and act to stabilize those that are biologically active.