Prognostic influence of mechanical cardiopulmonary resuscitation on survival in patients with out-of-hospital cardiac arrest undergoing ECPR on VA-ECMO.

Introduction: The use of venoarterial extracorporeal membrane oxygenation (VA-ECMO) in extracorporeal cardiopulmonary resuscitation (ECPR) in selected patients after out-of-hospital cardiac arrest (OHCA) is an established method if return of spontaneous circulation cannot be achieved. Automated chest compression devices (ACCD) facilitate transportation of patients under ongoing CPR and might improve outcome. We thus sought to evaluate prognostic influence of mechanical CPR using ACCD in patients presenting with OHCA treated with ECPR including VA-ECMO. Methods: We retrospectively analyzed data of 171 consecutive patients treated for OHCA using ECPR in our cardiac arrest center from the years 2016 to 2022. A Cox proportional hazards model was used to identify characteristics related with survival. Results: Of the 171 analyzed patients (84% male, mean age 56 years), 12% survived the initial hospitalization with favorable neurological outcome. The primary reason for OHCA was an acute coronary event (72%) followed by primary arrhythmia (9%) and non-ischemic cardiogenic shock (6.7%). In most cases, the collapse was witnessed (83%) and bystander CPR was performed (83%). The median time from collapse to VA-ECMO was 81 min (Q1: 69 min, Q3: 98 min). No survival benefit was seen for patients resuscitated using ACCD. Patients in whom an ACCD was used presented with overall longer times from collapse to ECMO than those who were resuscitated manually [83 min (Q1: 70 min, Q3: 98 min) vs. 69 min (Q1: 57 min, Q3: 84 min), p = 0.004]. Conclusion: No overall survival benefit of the use of ACCD before ECPR is established was found, possibly due to longer overall CPR duration. This may arguably be because of the limited availability of ACCD in pre-clinical paramedic service at the time of observation. Increasing the availability of these devices might thus improve treatment of OHCA, presumably by providing efficient CPR during transportation and transfer.

Gender disparities in patients undergoing extracorporeal cardiopulmonary resuscitation.

Introduction: The use of venoarterial extracorporeal membrane oxygenation (VA-ECMO) in extracorporeal cardiopulmonary resuscitation (eCPR) has emerged as a treatment option for selected patients who are experiencing refractory cardiac arrest (CA). In the light of increasing availability, the analyses of outcome-relevant predisposing characteristics are of growing importance. We evaluated the prognostic influence of gender in patients presenting with out-of-hospital cardiac arrest (OHCA) treated with eCPR. Methods: We retrospectively analysed the data of 377 consecutive patients treated for OHCA using eCPR in our cardiac arrest centre from January 2016 to December 2022. The primary outcome was defined as the survival of patients until they were discharged from the hospital, with a favourable neurological outcome [cerebral performance category (CPC) score of ≤2]. Statistical analyses were performed using baseline comparison, survival analysis, and multivariable analyses. Results: Out of the 377 patients included in the study, 69 (21%) were female. Female patients showed a lower prevalence rate of pre-existing coronary artery disease (48% vs. 75%, p < 0.001) and cardiomyopathy (17% vs. 34%, p = 0.01) compared with the male patients, while the mean age and prevalence rate of other cardiovascular risk factors were balanced. The primary reason for CA differed significantly (female: coronary event 45%, pulmonary embolism 23%, cardiogenic shock 17%; male: coronary event 70%, primary arrhythmia 10%, cardiogenic shock 10%; p = 0.001). The prevalence rate of witnessed collapse (97% vs. 86%; p = 0.016) and performance of bystander CPR (94% vs. 85%; p = 0.065) was higher in female patients. The mean time from collapse to the initiation of eCPR did not differ between the two groups (77 ± 39 min vs. 80 ± 37 min; p = 0.61). Overall, female patients showed a higher percentage of neurologically favourable survival (23% vs. 12%; p = 0.027) despite a higher prevalence of procedure-associated bleeding complications (33% vs. 16%, p = 0.002). The multivariable analysis identified a shorter total CPR duration (p = 0.001) and performance of bystander CPR (p = 0.03) to be associated with superior neurological outcomes. The bivariate analysis showed relevant interactions between gender and body mass index (BMI). Conclusion: Our analysis suggests a significant survival benefit for female patients who obtain eCPR, possibly driven by a higher prevalence of witnessed collapse and bystander CPR. Interestingly, the impact of patient age and BMI on neurologically favourable outcome was higher in female patients than in male patients, warranting further investigation.

Barriers to rejecting junk food sponsorship in sport-a formative evaluation using concept mapping.

OBJECTIVES: Energy-dense, nutrient-poor food and drink ('junk food') brands sponsoring sport is a growing public health concern. This study explored sports administrators' perceptions of the barriers to rejecting junk food sponsorship. STUDY DESIGN: This study used concept mapping. METHODS: The Concept Systems Global MAX™ web platform was used to collect and analyse data from 29 sports administrators across all levels of sport in Victoria, Australia. RESULTS: Brainstorming generated 33 barriers to rejecting junk food sponsorship. After the barriers were synthesised and edited, participants sorted and rated 32 barriers. Multidimensional scaling and hierarchical cluster analysis identified a four-cluster solution: community attitudes and values (seven barriers); junk food is the easy sell (retail; five barriers); financial viability (16 barriers); and organisational capability (policy and governance; four barriers). The financial viability barriers were rated the most important (mean = 3.65 of 5) and the hardest to overcome (1.42). The organisational capability (policy and governance) barriers were rated the least important (2.14) and the easiest to overcome (3.20). CONCLUSIONS: Sports administrators clearly perceive that rejecting junk food sponsorship could place significant financial strain on their organisations. There appears to be considerable scope to build the capacity of sporting organisations to rejecting junk food sponsorship. Despite the literature indicating that most parents think junk food companies are not suitable sponsors, sports administrators perceive that there is a broad public acceptance of junk food sponsorship in sport. The fact that sports administrators perceive a link between junk food sponsorship and the lack of healthy options at club canteens and venue food outlets adds an additional, not previously identified, level of complexity to the junk food sponsorship in sport debate.

Arrhythmia and exercise intolerance in Fontan patients: current status and future burden.

BACKGROUND: Long-term survival after the Fontan procedure shows excellent results but is associated with a persistent risk of arrhythmias and exercise intolerance. We aimed to analyze the current burden of clinically relevant arrhythmia and severe exercise intolerance in Danish Fontan patients and furthermore, to estimate the future burden from analysis of mortality and the current burden related to age. METHODS: All Danish citizens with Fontan completion from 1981 to 2009 were identified (n=235). Surviving patients performed exercise test, Holter monitoring, echocardiography, pulmonary function test, and blood sampling and medical history was retrieved from medical records. RESULTS: Twenty-six (11%) patients died or had heart transplantation (HTx) after a mean (± SD) post-Fontan follow-up of 8.3 ± 5.7 years. Excluding perioperative deaths (n=8), a linear probability of HTx-free survival was observed and estimated to 99.1% per year. Prevalence of clinically relevant arrhythmia and severe exercise intolerance increased significantly with age and was found in 32% and 85% of patients ≥ 20 years, respectively. Thus, from survival data and logistic regression models the future prevalence of patients, clinically relevant arrhythmia and severe exercise intolerance were estimated, revealing a considerable augmentation. Furthermore, resting and maximum cardiac index, resting stroke volume index and pulmonary diffusing capacity decreased significantly with age while diastolic and systolic ventricular function was unchanged. CONCLUSIONS: The prevalence of clinically relevant arrhythmia and severe exercise intolerance increased significantly with age in Danish Fontan patients. The future Fontan burden was estimated showing an increase in the prevalence of older patients, clinically relevant arrhythmia, and severe exercise intolerance.

Univentricular hearts in Denmark 1977 to 2009: incidence and survival.

BACKGROUND: The incidence of children born with functional univentricular heart (UVH) and their prognosis presumably changed substantially in recent years. This is due to introduction of fetal echocardiography and potential termination of pregnancy (TOP) when UVH is diagnosed (UVH TOP), and to improvements in treatment. We aimed to explore changes in incidence, to estimate changes in survival, and to describe predictors of mortality in UVH patients. METHODS: Using a population-based design we identified all UVH cases in Denmark from 1977 to 2009. RESULTS: 703 UVH live births and 106 UVH TOP were identified. A dramatic decrease in birth incidence of UVH patients and a corresponding increase in UVH TOP was observed in recent years. Mean incidence rate of UVH (live births and UVH TOP) was 0.39 per 1000 births. In adjusted analysis survival improved significantly from birth era 1977-1989 to 1990-1999 (HR 2.65, 95% confidence interval (CI), 2.06-3.42) but not significantly from 1990-1999 to 2000-2009 (HR 0.77, 95% CI, 0.57-1.05). In the birth era 2000-2009, the lowest five-year survival was seen with hypoplastic left heart syndrome (HLHS) (18.8%), whereas the best survival was seen with tricuspid atresia (79.8%). Adjusted risk of death was 7.3 times higher in the HLHS group compared to the tricuspid atresia group (95% CI, 3.94-13.47). CONCLUSIONS: This study demonstrates a dramatic decrease in birth incidence of UVH patients most probably due to a corresponding increase in UVH TOP. Despite survival improved after introduction of Fontan surgery, survival has not improved significantly during the last 20years.

Thromboembolic complications in Fontan patients: population-based prevalence and exploration of the etiology.

After the Fontan procedure, patients face an increased risk for thromboembolic events (TE). The etiology for this increased thrombogenecity is incompletely understood. This study aimed to determine the prevalence of TE in Danish Fontan patients and to bring new insights into the etiology of TE. Using a population-based design, we retrospectively identified all TEs in 210 Fontan patients. Whole blood assays (thromboelastography, thromboelastography functional fibrinogen and Multiplate) reflecting global hemostasis, clot strength and platelet aggregation were analyzed prospectively in 112 patients and plasma was analyzed in 76 patients for biomarkers reflecting endothelial-, glycocalyx-, platelet-, and fibrinolysis function (histone-complexed DNA fragments, Protein C, soluble CD40 ligand, soluble thrombomodulin, syndecan-1, tissue-type plasminogen activator). The results were compared in groups stratified according to age, antithrombotic therapy, TE, and glycocalyx degradation (syndecan-1 < or ≥ median). Correlation between biomarkers and demographic-, anatomical-, clinical- and biochemical parameters was investigated. The prevalence of TE was 8.1 % after a mean follow-up of 8.4 years. None of the stratified groups demonstrated evidence of hypercoagulability in the whole blood assays and no unexpected significant differences were found between the groups. All biomarkers, except protein C, correlated with one another and after stratification of glycocalyx degradation only syndecan-1 levels ≥ median correlated with other biomarkers. The prevalence of TEs was 8.1 % after mean follow-up of 8.4 years. Overall, the hemostatic profile appeared normal, however, in a subset of patients, evidence of some endothelial activation/damage including glycocalyx degradation and fibrinolysis was found, identifying a potentially more thrombogenic group.

On-chip magnetic bead microarray using hydrodynamic focusing in a passive magnetic separator.

Implementing DNA and protein microarrays into lab-on-a-chip systems can be problematic since these are sensitive to heat and strong chemicals. Here, we describe the functionalization of a microchannel with two types of magnetic beads using hydrodynamic focusing combined with a passive magnetic separator with arrays of soft magnetic elements. The soft magnetic elements placed on both sides of the channel are magnetized by a relatively weak applied external magnetic field (21 mT) and provide magnetic field gradients attracting magnetic beads. Flows with two differently functionalized magnetic beads and a separating barrier flow are introduced simultaneously at the two channel sides and the centre of the microfluidic channel, respectively. On-chip experiments with fluorescence labeled beads demonstrate that the two types of beads are captured at each of the channel sidewalls. On-chip hybridization experiments show that the microfluidic systems can be functionalized with two sets of beads carrying different probes that selectively recognize a single base pair mismatch in target DNA. By switching the places of the two types of beads it is shown that the microsystem can be cleaned and functionalized repeatedly with different beads with no cross-talk between experiments.

[Percutaneous revascularization of multivessel coronary disease using stents - a multicenter, prospective study]. / Perkutane Revaskularisation der koronaren Mehrgefässerkrankung nach Einführung von Stents - Eine multizentrische, prospektive Studie.

BACKGROUND AND OBJECTIVE: Symptomatic patients with multivessel coronary disease (MVD) benefit from both coronary artery bypass grafting (CABG) and percutaneous coronary angioplasty (PTCA). The >>German Angioplasty Bypass Investigation<< (GABI-I) trial randomized patients to one of these treatment strategies between 1986 and 1991. In order to evaluate the impact of current technology, in particular coronary stents, the GABI-II trial was initiated, which in 1996 and 1997 prospectively enrolled patients according to the initial GABI-I criteria. PATIENTS AND METHODS: Into the study 136 consecutive patients (108 men, 28 women; 63 +/- 12 years) were included. Patients from GABI-I served as controls. RESULTS: A mean of 2.1 +/- 0.5 vessels were treated per patient (vs. 1.9 +/- 0.5 vessels in the PTCA arm of GABI-I) and 63 % of the lesions were covered with stents. With respect to the primary endpoint less patients remained with a CCS class III or IV in GABI-II after 12 months (1,5 % vs. 8 % in the PTCA arm of GABI-I, p<0,01). No patient required emergency or urgent bypass operation in GABI-II (vs. 9 % in GABI-I, p < 0.01). After 12 months, 8 % of the patients were sent for bypass surgery (CABG) vs. 21 % in GABI-I (p < 0.001), and 20 % (vs. 23 % in GABI-I) of the patients underwent Re-PTCA. The percentage of patients without reinterventions was 72 % vs. 56 % in GABI-I (p < 0.01), but remained lower compared to patients randomized to CABG in GABI-I (94 %, p < 0.001). CONCLUSION: PTCA in patients with MVD is still associated with a higher reintervention rate as compared with CABG. However, in contrast to angioplasty a decade ago, PTCA in conjunction with stents significantly lowered the need for subsequent revascularization, which was mainly driven by the reduced necessity for bypass surgery.

Modeling the bacterial photosynthetic reaction center. 4. The structural, electrochemical, and hydrogen-bonding properties of 22 mutants of Rhodobacter sphaeroides.

Site-directed mutagenesis has been employed by a number of groups to produce mutants of bacterial photosynthetic reaction centers, with the aim of tuning their operation by modifying hydrogen-bond patterns in the close vicinity of the "special pair" of bacteriochlorophylls P identical with P(L)P(M). Direct X-ray structural measurements of the consequences of mutation are rare. Attention has mostly focused on effects on properties such as carbonyl stretching frequencies and midpoint potentials to infer indirectly the induced structural modifications. In this work, the structures of 22 mutants of Rhodobacter sphaeroides have been calculated using a mixed quantum-mechanical molecular-mechanical method by modifying the known structure of the wild type. We determine (i) the orientation of the 2a-acetyl groups in the wild type, FY(M197), and FH(M197) series mutants of the neutral and oxidized reaction center, (ii) the structure of the FY(M197) mutant and possible water penetration near the special pair, (iii) that significant protein chain distortions are required to assemble some M160 series mutants (LS(M160), LN(M160), LQ(M160), and LH(M160) are considered), (iv) that there is competition for hydrogen-bonding between the 9-keto and 10a-ester groups for the introduced histidine in LH(L131) mutants, (v) that the observed midpoint potential of P for HL(M202) heterodimer mutants, including one involving also LH(M160), can be correlated with the change of electrostatic potential experienced at P(L), (vi) that hydrogen-bond cleavage may sometimes be induced by oxidation of the special pair, (vii) that the OH group of tyrosine M210 points away from P(M), and (viii) that competitive hydrogen-bonding effects determine the change in properties of NL(L166) and NH(L166) mutants. A new technique is introduced for the determination of ionization energies at the Koopmans level from QM/MM calculations, and protein-induced Stark effects on vibrational frequencies are considered.

Membrane-covered stents: a new treatment strategy for saphenous vein graft lesions.

The restenosis rate after stenting of lesions in aortocoronary venous bypass grafts still has to be considered unsatisfactorily high. We investigated a new stent design characterized by an expandable polytetrafluorethylene (PTFE) membrane in between two layers of struts. Five consecutive male patients (age 70 +/- 6 years) were followed prospectively who presented with at least two de novo lesions in different grafts 13 +/- 3 years after bypass surgery. A total of 11 lesions were treated located in grafts anastomosed to the circumflex (n = 3), to the LAD (n = 7), and to the right coronary artery (n = 1). Within the same procedure, every patient received membrane-covered stents (n = 6) and conventional stents (n = 5) in either of their lesions. All patients underwent successful interventions. The minimal luminal diameter increased from 1.0 +/- 0.5 to 2.9 +/- 0.6 mm in lesions treated by the membrane-covered stents and from 0.8 +/- 0.4 to 2.4 +/- 0.7 mm in the lesions treated by conventional stents. During follow-up, four out of five patients required angioplasty for in-stent restenosis of lesions covered by a conventional stent, whereas no patient underwent revascularization for a lesion treated by a membrane-covered device. The mean minimal luminal diameter of lesions covered by a conventional stent decreased by 42% to 1.4 +/- 0.6 mm; the mean minimal luminal diameter of the lesions treated by a stent graft declined by 9% to 2.8 +/- 0.6 mm (P < 0.05). This series of intraindividual comparisons suggests that membrane-covered stents may have the power to reduce in-stent restenosis in obstructed aortocoronary venous bypass grafts.

Static and dynamic problems in spastic cerebral palsy.

1. Analysis of the static and dynamic conditions in spastic cerebral palsy leads to the conclusion that contractures of the hip, knee and ankle should be released from above downwards, and that the full benefit is obtained only when all contractures have been corrected.
2. Only when these joints are mobilized by removal of the significant contractures is the spastic patient able, despite other handicaps, to sit, stand and walk with the least hindrance and the least cosmetic fault.
3. In our Danish experience, inadequate or poor results from the orthopaedic surgery of cerebral palsy are mainly due to the fact that patients with contractures are operated on too little and too late, or not in the correct sequence.

Modeling the bacterial photosynthetic reaction center 3: interpretation of effects of site-directed mutagenesis on the special-pair midpoint potential.

Interpretation of changes in midpoint potential of the "special pair" in bacterial photosynthetic reaction centers caused by site-directed mutagenesis is discussed in terms of a simple tight-binding model which relates them to concomitant variations in spin distribution between the two bacteriochlorophyll molecules of the special pair. Our analysis improves on previous similar ones by Allen and co-workers [Artz, K., Williams, J. C., Allen, J. P., Lendzian, F., Rautter, J., and Lubitz, W. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 13582; Ivancich, A., Artz, K., Williams, J. C., Allen, J. P., and Mattioli, T. A. (1998) Biochemistry 37, 11812] in that it is both more complete, including electron-phonon coupling, and more accurate. It is applied to analyze data for a series of M160 mutants of Rhodobacter sphaeroides, yielding a value of 0.18+/-0.03 eV for the electronic coupling energy between the highest occupied levels of the two bacteriochlorophylls in the wild-type and a value of the energy offset E(o) between the highest occupied molecular orbitals of the L and M bacteriochlorophylls of 0.14+/-0.03 eV. For a mutant in which the electron hole in the special pair cation is located entirely on the reactive (L) side, a potential of 641+/-30 mV with respect to the normal hydrogen electrode is predicted. This agrees well with the average value ca. 650 mV observed for the heterodimer mutant HL(M202) in which the bacteriochlorophyll on the unreactive M side has been replaced by a bacteriopheophytin, causing extensive charge localization. However, the deduced coupling is found to be very sensitive to small changes in the assumptions used in the model, and various important chemical effects remain to be included.

Six-month clinical and angiographic outcome after successful excimer laser angioplasty for in-stent restenosis.

OBJECTIVES: This study evaluated the clinical and angiographic six-month follow-up after excimer laser coronary angioplasty (ELCA) for restenosed coronary stents. BACKGROUND: Excimer laser coronary angioplasty has recently been shown to be safe and efficient for the treatment of in-stent restenosis. METHODS: Ninety-six consecutive patients successfully treated with ELCA within 141 stents were included in a six-month clinical and angiographic follow-up. RESULTS: During follow-up there was one sudden death and one patient with documented myocardial infarction. Angina pectoris classified as > or = Canadian Cardiovascular Society II reoccurred in 49 patients. Follow-up angiography was obtained in 89 patients (93%) with 133 stents. Quantitative coronary angiography revealed a mean diameter stenosis of 77 +/- 10% before intervention, 41 +/- 12% after laser treatment and 11% +/- 12% after adjunctive percutaneous transluminal coronary angioplasty (p < 0.001). Six months after ELCA the mean diameter stenosis had increased to 60 +/- 26% (p < 0.001). A > or =50% diameter stenosis was present in 48 patients (54%); in 24 of these patients diameter stenosis was > or =70%. Total occlusions occurred in an additional 10 patients (11%). There was a trend toward an increased recurrent restenosis rate in patients with diabetes mellitus and long lesions or total occlusions (p = 0.059). Forty-eight patients (50%) received medical treatment after six months. Reinterventions were necessary in 30 patients (31%), and coronary artery bypass surgery was performed in 17 patients (18%). Event-free survival was 50%. CONCLUSIONS: Excimer laser angioplasty for in-stent restenosis was associated with a high incidence of recurrent restenosis in this group of patients, suggesting that this technique is unlikely to reduce recurrent in-stent restenosis and that other approaches are necessary.

Membrane-covered stents for the treatment of aortocoronary vein graft disease.

We report of a 78-year-old patient who underwent angioplasty for two de novo lesions in different aortocoronary venous bypass grafts. Whereas one lesion was treated by conventional stents, two adjacent lesions in a second bypass graft were covered by two newly designed stents with a polytetrafluorethylene (PTFE) membrane in between two layers of struts. The patient was first readmitted at month 5 with recurrent angina pectoris. High-grade in-stent restenosis of the lesion covered by the conventional stent was diagnosed, whereas the lesions treated by the membrane-covered stent revealed no restenosis. The patient underwent subsequent balloon angioplasty and was readmitted with unstable angina 8 months after the initial procedure. The angiogram revealed subtotal occlusion of the vein graft due to diffuse in-stent restenosis within the conventional stent. Undergoing re-PTCA, the patient received an additional membrane-covered stent placed over the proximal device. For the following 7 months, the patient stayed asymptomatic and underwent elective reangiography, which revealed a satisfying long-term result with no relevant luminal loss at the target site. This intraindividual comparison of stents in aortocoronary vein grafts suggests that PTFE membrane-covered stents might reduce restenosis as compared to conventional stents in venous bypass grafts. Furthermore, these stents potentially represent a new treatment strategy to fight in-stent restenosis in selected lesions.

Initial experience with a hydrophilic-coated guidewire for recanalization of chronic coronary occlusions.

Chronic coronary occlusions are still a therapeutic challenge to the interventional cardiologist. New techniques such as laser wire have improved recanalization rates, but outcomes are still far from satisfactory. We report the results of a nonrandomized single-center investigation using a hydrophilic-coated guidewire (Terumo Crosswire). Between September 1996 and September 1998, 107 chronic occlusions in 106 patients were approached when previous attempts with conventional guidewires failed. Median occlusion duration in these cases was 4 months (range, 0.5-122); mean occlusion length was 19 +/- 11 mm (range, 5-60). Forty-five (42%) of these attempts were successful. Attempts were successful in 42% in the left anterior descending artery, in 30% in the left circumflex artery, in 48% in the right coronary artery, and in 43% in coronary artery bypass grafts. Success rates ranged from 56% for occlusions of less than 4-month duration to 18% for occlusions of more than 36-month duration. The success rate in TIMI 1-flow lesions was significantly higher than in TIMI 0 flow lesions, 85% vs. 36%. In a multivariate regression analysis, TIMI flow grade and occlusion age were independent predictors of success. There were no deaths or Q-wave myocardial infarctions; two cases of hemopericardium were treated successfully. In five cases, pericardial contrast staining due to vessel perforation occurred. Our results indicate that the Crosswire is an effective tool in the treatment of chronic coronary occlusions, even when recanalization attempts with conventional guidewires fail. Cathet. Cardiovasc. Intervent. 49:45-50, 2000.

Hypermutation in derepressed operons of Escherichia coli K12.

This article presents evidence that starvation for leucine in an Escherichia coli auxotroph triggers metabolic activities that specifically target the leu operon for derepression, increased rates of transcription, and mutation. Derepression of the leu operon was a prerequisite for its activation by the signal nucleotide, guanosine tetraphosphate, which accumulates in response to nutritional stress (the stringent response). A quantitative correlation was established between leuB mRNA abundance and leuB- reversion rates. To further demonstrate that derepression increased mutation rates, the chromosomal leu operon was placed under the control of the inducible tac promoter. When the leu operon was induced by isopropyl-D-thiogalactoside, both leuB mRNA abundance and leuB- reversion rates increased. These investigations suggest that guanosine tetraphosphate may contribute as much as attenuation in regulating leu operon expression and that higher rates of mutation are specifically associated with the derepressed leu operon.