Stat3: a STAT family member activated by tyrosine phosphorylation in response to epidermal growth factor and interleukin-6

The STAT family of proteins carries out a dual function: signal transduction and activation of transcription. A new family member, Stat3, becomes activated through phosphorylation on tyrosine as a DNA binding protein in response to epidermal growth factor (EGF) and interleukin-6 (IL-6) but not interferon gamma (IFN-gamma). It is likely that this phosphoprotein forms homodimers as well as heterodimers with the first described member of the STAT family, Stat91 (renamed Stat1 alpha), which is activated by the IFNs and EGF. Differential activation of different STAT proteins in response to different ligands should help to explain specificity in nuclear signaling from the cell surface.

Maximal activation of transcription by Stat1 and Stat3 requires both tyrosine and serine phosphorylation

Stat1 and Stat3 are latent transcriptional factors activated initially through phosphorylation on single tyrosine residues induced by cytokine and growth factor occupation of cell surface receptors. Here we show that phosphorylation on a single serine (residue 727) in each protein is also required for maximal transcriptional activity. Both cytokines and growth factors are capable of inducing the serine phosphorylation of Stat1 and Stat3. These experiments show that gene activation by Stat1 and Stat3, which obligatorily require tyrosine phosphorylation to become active, also depends for maximal activation on one or more of the many serine kinases.

Choice of STATs and other substrates specified by modular tyrosine-based motifs in cytokine receptors

Many members of the cytokine receptor superfamily initiate intracellular signaling by activating members of the Jak family of tyrosine kinases. Activation of the same Jaks by multiple cytokines raises the question of how these cytokines activate distinct intracellular signaling pathways. Selection of particular substrates--the transcriptional activator Stat3 and protein tyrosine phosphatase PTP1D--that characterize responses to the ciliary neurotrophic factor-interleukin-6 cytokine family depended not on which Jak was activated, but was instead determined by specific tyrosine-based motifs in the receptor components--gp130 and LIFR--shared by these cytokines. Further, these tyrosine-based motifs were modular, because addition of a Stat3-specifying motif to another cytokine receptor, that for erythropoietin, caused it to activate Stat3 in a ligand-dependent fashion.

First-line erlotinib versus gemcitabine/cisplatin in patients with advanced EGFR mutation-positive non-small-cell lung cancer: analyses from the phase III, randomized, open-label, ENSURE study

BACKGROUND

The phase III, randomized, open-label ENSURE study (NCT01342965) evaluated first-line erlotinib versus gemcitabine/cisplatin (GP) in patients from China, Malaysia and the Philippines with epidermal growth factor receptor (EGFR) mutation-positive non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

Patients ≥18 years old with histologically/cytologically confirmed stage IIIB/IV EGFR mutation-positive NSCLC and Eastern Cooperative Oncology Group performance status 0-2 were randomized 1:1 to receive erlotinib (oral; 150 mg once daily until progression/unacceptable toxicity) or GP [G 1250 mg/m(2) i.v. days 1 and 8 (3-weekly cycle); P 75 mg/m(2) i.v. day 1, (3-weekly cycle) for up to four cycles]. Primary end point: investigator-assessed progression-free survival (PFS). Other end points include objective response rate (ORR), overall survival (OS), and safety.

RESULTS

A total of 217 patients were randomized: 110 to erlotinib and 107 to GP. Investigator-assessed median PFS was 11.0 months versus 5.5 months, erlotinib versus GP, respectively [hazard ratio (HR), 0.34, 95% confidence interval (CI) 0.22-0.51; log-rank P < 0.0001]. Independent Review Committee-assessed median PFS was consistent (HR, 0.42). Median OS was 26.3 versus 25.5 months, erlotinib versus GP, respectively (HR, 0.91, 95% CI 0.63-1.31; log-rank P = .607). ORR was 62.7% for erlotinib and 33.6% for GP. Treatment-related serious adverse events (AEs) occurred in 2.7% versus 10.6% of erlotinib and GP patients, respectively. The most common grade ≥3 AEs were rash (6.4%) with erlotinib, and neutropenia (25.0%), leukopenia (14.4%), and anemia (12.5%) with GP.

CONCLUSION

These analyses demonstrate that first-line erlotinib provides a statistically significant improvement in PFS versus GP in Asian patients with EGFR mutation-positive NSCLC (NCT01342965).

Interleukin 12 signaling in T helper type 1 (Th1) cells involves tyrosine phosphorylation of signal transducer and activator of transcription (Stat)3 and Stat4

Interleukin 12 (IL-12) initiates the differentiation of naive CD4+ T cells to T helper type 1 (Th1) cells critical for resistance to intracellular pathogens such as Leishmania major. To explore the basis of IL-12 action, we analyzed induction of nuclear factors in Th1 cells. IL-12 selectively induced nuclear DNA-binding complexes that contained Stat3 and Stat4, recently cloned members of the family of signal transducers and activators of transcription (STATs). While Stat3 participates in signaling for several other cytokines, Stat4 was not previously known to participate in the signaling pathway for any natural ligand. The selective activation of Stat4 provides a basis for unique actions of IL-12 on Th1 development. Thus, this study presents the first identification of the early events in IL-12 signaling in T cells and of ligand activation of Stat4.

Diffraction enhanced x-ray imaging

Diffraction enhanced imaging is a new x-ray radiographic imaging modality using monochromatic x-rays from a synchrotron which produces images of thick absorbing objects that are almost completely free of scatter. They show dramatically improved contrast over standard imaging applied to the same phantom. The contrast is based not only on attenuation but also the refraction and diffraction properties of the sample. This imaging method may improve image quality for medical applications, industrial radiography for non-destructive testing and x-ray computed tomography.

Stat3 and Stat4: members of the family of signal transducers and activators of transcription

The deduced amino acid sequence of two members of the signal transducers and activators of transcription (STAT) family from the mouse are described. Comparison with the deduced protein sequence of the two previously described genes (Stat91 and Stat113), discovered because of their activation as transcription factors after interferon-induced tyrosine phosphorylation, shows several highly conserved regions, including the putative SH3 and SH2 domains. The conserved amino acid stretches likely point to conserved domains that enable these proteins to carry out the several required functions they are known and proposed to carry out. While Stat1 and Stat3 are widely expressed, Stat4 expression is restricted to testis, thymus, and spleen. Antiserum to Stat3 detects a major approximately 92-kDa protein and a minor approximately 89-kDa protein, while antiserum to Stat4 precipitates one major protein of approximately 89 kDa.

Evaluation of prognostic criteria for determining hospice eligibility in patients with advanced lung, heart, or liver disease. SUPPORT Investigators. Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatments

CONTEXT

Many individuals involved with care of the dying advocate expanding access to hospice care for persons with advanced lung, heart, or liver disease. However, to be eligible, these patients generally must have a prognosis for survival of less than 6 months.

OBJECTIVE

To test the ability of currently available criteria to identify a population with a survival prognosis of 6 months or less among seriously ill hospitalized patients with 1 of 3 commonly fatal chronic diseases.

DESIGN

Validation study using data from the Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatments (SUPPORT) phase 1 (June 1989-June 1991) and phase 2 (January 1992-January 1994), with a 6-month follow-up.

SETTING AND PATIENTS

Consecutive sample of 2607 seriously ill patients from 5 US medical centers who were hospitalized with chronic obstructive pulmonary disease, congestive heart failure, or end-stage liver disease, and who survived to hospital discharge.

MAIN OUTCOME MEASURES

Descriptive and operating characteristics of 5 general and 2 disease-specific clinical criteria for identifying patients with a survival prognosis of 6 months or less, and 3 sets of combination criteria (broad, intermediate, and narrow inclusion) aimed at providing low, medium, and high thresholds for hospice eligibility based on National Hospice Organization guidelines.

RESULTS

Seventy-five percent of the sample survived more than 6 months after hospital discharge; 44% expressed a preference for palliative care. Broad inclusion criteria identified 923 patients eligible for hospice care, of whom 70% survived longer than 6 months. Intermediate inclusion criteria identified 300 patients, of whom 65% survived longer than 6 months. Narrow inclusion criteria identified 19 patients, of whom 53% survived longer than 6 months. Sensitivities and specificities of the combination criteria were 41.7% and 66.7% (broad inclusion), 16.2% and 90.1 % (intermediate inclusion), and 1.4% and 99.5% (narrow inclusion), respectively.

CONCLUSIONS

These data indicate that for seriously ill hospitalized patients with advanced chronic obstructive pulmonary disease, congestive heart failure, or end-stage liver disease, recommended clinical prediction criteria are not effective in identifying a population with a survival prognosis of 6 months or less.

Dying with lung cancer or chronic obstructive pulmonary disease: insights from SUPPORT. Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatments

CONTEXT

Many are calling for patients with advanced chronic obstructive pulmonary disease (COPD) to receive hospice care, but the traditional hospice model may be insufficient.

OBJECTIVE

To compare the course of illness and patterns of care for patients with non-small cell lung cancer and severe COPD.

DESIGN

Prospective cohort study of seriously ill, hospitalized adults.

SETTING

Five teaching hospitals in the United States.

PATIENTS

Patients with Stage III or IV non-small cell lung cancer (n = 939) or acute exacerbation of severe COPD (n = 1008).

MAIN OUTCOME MEASURES

Patients' preferences for pattern of care and for ventilator use; symptoms; life-sustaining interventions; and survival prognoses.

RESULTS

Sixty percent in each group wanted comfort-focused care; 81% with lung cancer and 78% with COPD were extremely unwilling to have mechanical ventilation indefinitely. Severe dyspnea occurred in 32% of patients with lung cancer and 56% of patients with COPD and severe pain in 28 % of patients with lung cancer and 21% of patients with COPD. Patients with COPD who died during index hospitalization were more likely than patients with lung cancer to receive mechanical ventilation (70.4% vs 19.8%), tube feeding (38.7% vs 18.5%), and cardiopulmonary resuscitation (25.2% vs 7.8%). Mechanical ventilation had greater short term effectiveness in patients with COPD, based on survival to hospital discharge (76% vs 38%). Patients with COPD maintained higher median 2-month and 6-month survival prognoses, even days before death.

CONCLUSIONS

Hospitalized patients with lung cancer or COPD preferred comfort-focused care, yet dyspnea and pain were problematic in both groups. Patients with COPD were more often treated with life-sustaining interventions, and short-term effectiveness was comparatively better than in patients with lung cancer. In caring for patients with severe COPD, consideration should be given to implementing palliative treatments more aggressively, even while remaining open to provision of life-sustaining interventions.

Small-molecule RORγt antagonists inhibit T helper 17 cell transcriptional network by divergent mechanisms

We identified three retinoid-related orphan receptor gamma t (RORγt)-specific inhibitors that suppress T helper 17 (Th17) cell responses, including Th17-cell-mediated autoimmune disease. We systemically characterized RORγt binding in the presence and absence of drugs with corresponding whole-genome transcriptome sequencing. RORγt acts as a direct activator of Th17 cell signature genes and a direct repressor of signature genes from other T cell lineages; its strongest transcriptional effects are on cis-regulatory sites containing the RORα binding motif. RORγt is central in a densely interconnected regulatory network that shapes the balance of T cell differentiation. Here, the three inhibitors modulated the RORγt-dependent transcriptional network to varying extents and through distinct mechanisms. Whereas one inhibitor displaced RORγt from its target loci, the other two inhibitors affected transcription predominantly without removing DNA binding. Our work illustrates the power of a system-scale analysis of transcriptional regulation to characterize potential therapeutic compounds that inhibit pathogenic Th17 cells and suppress autoimmunity.

Characterization of a murine monoclonal antibody to Cryptococcus neoformans polysaccharide that is a candidate for human therapeutic studies

The murine monoclonal antibody (MAb) 18B7 [immunoglobulin G1(kappa)] is in preclinical development for treatment of Cryptococcus neoformans infections. In anticipation of its use in humans, we defined the serological and biological properties of MAb 18B7 in detail. Structural comparison to the related protective MAb 2H1 revealed conservation of the antigen binding site despite several amino acid differences. MAb 18B7 was shown by immunofluorescence and agglutination studies to bind to all four serotypes of C. neoformans, opsonize C. neoformans serotypes A and D, enhance human and mouse effector cell antifungal activity, and activate the complement pathway leading to deposition of complement component 3 (C3) on the cryptococcal capsule. Administration of MAb 18B7 to mice led to rapid clearance of serum cryptococcal antigen and deposition in the liver and spleen. Immunohistochemical studies revealed that MAb 18B7 bound to capsular glucuronoxylomannan in infected mouse tissues. No reactivity of MAb 18B7 with normal human, rat, or mouse tissues was detected. The results show that both the variable and constant regions of MAb 18B7 are biologically functional and support the use of this MAb in human therapeutic trials.

Computed tomography of x-ray index of refraction using the diffraction enhanced imaging method

Diffraction enhanced imaging (DEI) is a new, synchrotron-based, x-ray radiography method that uses monochromatic, fan-shaped beams, with an analyser crystal positioned between the subject and the detector. The analyser allows the detection of only those x-rays transmitted by the subject that fall into the acceptance angle (central part of the rocking curve) of the monochromator/analyser system. As shown by Chapman et al, in addition to the x-ray attenuation, the method provides information on the out-of-plane angular deviation of x-rays. New images result in which the image contrast depends on the x-ray index of refraction and on the yield of small-angle scattering, respectively. We implemented DEI in the tomography mode at the National Synchrotron Light Source using 22 keV x-rays, and imaged a cylindrical acrylic phantom that included oil-filled, slanted channels. The resulting 'refraction CT image' shows the pure image of the out-of-plane gradient of the x-ray index of refraction. No image artefacts were present, indicating that the CT projection data were a consistent set. The 'refraction CT image' signal is linear with the gradient of the refractive index, and its value is equal to that expected. The method, at the energy used or higher, has the potential for use in clinical radiography and in industry.

Differential activation of acute phase response factor/Stat3 and Stat1 via the cytoplasmic domain of the interleukin 6 signal transducer gp130. II. Src homology SH2 domains define the specificity of stat factor activation

Distinct yet overlapping sets of STAT transcription factors are activated by different cytokines. One example is the differential activation of acute phase response factor (APRF, also called Stat3) and Stat1 by interleukin 6 and interferon-gamma. Interleukin 6 activates both factors while, at least in human cells, interferon-gamma recruits only Stat1. Stat1 activation by interferon-gamma is mediated through a cytosolic tyrosine motif, Y440, of the interferon-gamma receptor. In an accompanying paper (Gerhartz, C., Heesel, B., Sasse, J., Hemmann, U., Landgraf, C., Schneider-Mergener, J., Horn, F., Heinrich, P. C., and Graeve, L. (1996) J. Biol. Chem. 271, 12991-12998), we demonstrated that two tyrosine motifs within the cytoplasmic part of the interleukin 6 signal transducer gp130 specifically mediate APRF activation while two others can recruit both APRF and Stat1. By expressing a series of Stat1/APRF domain swap mutants in COS-7 cells, we now determined which domains of Stat1 and APRF are involved in the specific recognition of phosphotyrosine motifs. Our data demonstrate that the SH2 domain is the sole determinant of specific STAT factor recruitment. Furthermore, the SH2 domain of Stat1 is able to recognize two unrelated types of phosphotyrosine motifs, one represented by the interferon-gamma receptor Y440DKPH peptide, and the other by two gp130 YXPQ motifs. By molecular modeling, we propose three-dimensional model structures of the Stat1 and APRF SH2 domains which allow us to explain the different binding preferences of these factors and to predict amino acids crucial for specific peptide recognition.

STAT3 activation by cytokines utilizing gp130 and related transducers involves a secondary modification requiring an H7-sensitive kinase

Ciliary neurotrophic factor, oncostatin M, leukemia-inhibitory factor, and interleukin 6 are related cytokines that initiate signaling by homodimerizing the signal-transducing receptor component gp130 or by heterodimerizing gp130 with a gp130-related receptor component. Receptor dimerization in turn activates receptor-associated kinases of the Jak/Tyk family, resulting in the rapid tyrosine phosphorylation of several intracellular proteins, including those of two members of the signal transducers and activators of transcription (STAT) family--STAT1 and STAT3. Here we show that all cytokines that utilize gp130 sequentially induce two distinct forms of STAT3 in all responding cells examined, with the two forms apparently differing because of a time-dependent secondary serine/threonine phosphorylation involving an H7-sensitive kinase. While both STAT3 forms bind DNA and translocate to the nucleus, the striking time-dependent progression from one form to the other implies other important functional differences between the two forms. Granulocyte colony-stimulating factor, which utilizes a receptor highly related to gp130, also induces these two forms of STAT3. In contrast to a number of other cytokines and growth factors, all cytokines using gp130 and related signal transducers consistently and preferentially induce the two forms of STAT3 as compared with STAT1; this characteristic STAT activation pattern is seen regardless of which Jak/Tyk kinases are used in a particular response, consistent with the notion that the receptor components themselves are the primary determinants of which STATs are activated.

Dying with cancer: patients' function, symptoms, and care preferences as death approaches

OBJECTIVE

To characterize the dying experience of patients with cancer over the last 6 months of life.

STUDY DESIGN

A retrospective analysis of the last 6 months of life among patients with colon cancer and non-small cell lung cancer enrolled in a prospective cohort study from June 1989 to June 1991 and from January 1992 to January 1994.

SETTING

Five geographically diverse tertiary care academic medical centers participating in the Study to Understand Patient Prognoses and Preferences for Outcomes and Risks of Treatments (SUPPORT) Project.

PARTICIPANTS

All patients enrolled in SUPPORT who had either colon cancer, metastatic to the liver or stage III or stage IV non-small cell lung cancer and died within 1 year of their index hospitalization. This report examines 316 of 520 patients with metastatic colon cancer and 747 of 939 patients with lung cancer enrolled in SUPPORT.

METHODS

Data were collected by interview and chart abstraction at several time points in the SUPPORT Project. To describe progression to death, we constructed four observational windows backward in time beginning with patients' date of death and ending with their date of entry into the SUPPORT Project or 6 months before their death, whichever came first: (1) 3 days before death, (2) 3 days to 1 month before death, (3) 1 month to 3 months before death, and (4) 3 months to 6 months before death. For each outcome, patients contributed information to all windows during which they had data collected. We describe the frequency distributions of each outcome over time and report tests for trend.

OUTCOME MEASURES

We examined several outcomes over time, including: percentage of days spent in a hospital; prognosis as measured by model-based prognostic estimates of 6-month survival; severity of illness as measured by the acute physiology score; functional status as measured by dependencies in activities of daily living (ADLs); severe physical and emotional symptoms, including pain, depression, and anxiety; patients' preferences for care; and the financial impact on patients' families.

RESULTS

The death rate within 1 year of study entry was high among patients with metastatic colon cancer and advanced non-small cell lung cancer enrolled in SUPPORT (61% and 80%, respectively). As patients with cancer progress toward death, their estimated 6-month prognosis decreases significantly and the severity of their disease worsens. Patients' functional status also declines significantly as they approach death, such that most patients have four or more impairments within the 3 days before death. Patients with cancer experience significantly more pain and confusion as death approaches. Severe pain is common; more than one-quarter of patients with cancer experience serious pain 3 to 6 months before death and more than 40% were in serious pain during their last 3 days of life. However, dying patients are only modestly depressed and anxious during their last 3 days of life. As death approaches, patients favor comfort measures over life-extension, and about two-thirds want to forego resuscitation within 3 days of death. Families of patients dying with cancer incurred significant financial burdens during the last 6 months of life, and much of this burden was already experienced by 3 to 6 months before death.

CONCLUSIONS

The last 6 months of life for patients with cancer is characterized by functional decline and poorly controlled severe pain and confusion. Although patients increasingly prefer comfort care as they near death, many die in severe pain. These findings highlight important opportunities to improve the quality of care at the end of life for patients dying with cancer.

Human breast cancer specimens: diffraction-enhanced imaging with histologic correlation--improved conspicuity of lesion detail compared with digital radiography

Seven breast cancer specimens were examined with diffraction-enhanced imaging at 18 keV with a silicon crystal with use of the silicon 333 reflection in Bragg mode. Images were compared with digital radiographs of the specimen, and regions of increased detail were identified. Six of the seven cases (86%) showed enhanced visibility of surface spiculation that correlated with histopathologic information, including extension of tumor into surrounding tissue.

Patients who want their family and physician to make resuscitation decisions for them: observations from SUPPORT and HELP. Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment. Hospitalized Elderly Longitudinal Project

OBJECTIVE

To determine the extent to which older or seriously ill inpatients would prefer to have their family and physician make resuscitation decisions for them rather than having their own stated preferences followed if they were unable to decide themselves.

DESIGN

Analysis of existing data from the Hospitalized Elderly Longitudinal Project (HELP) and the Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment (SUPPORT).

SETTING

Five teaching hospitals in the United States.

PARTICIPANTS

2203 seriously ill adult inpatients (SUPPORT) and 1226 older inpatients (HELP) who expressed preferences about resuscitation and about advance decision-making.

MEASURES

We used a logistic regression model to determine which factors predicted preferences for family and physician decision-making.

RESULTS

Of the 513 HELP patients in this analysis, 363 (70.8%) would prefer to have their family and physician make resuscitation decisions for them whereas 29.2% would prefer to have their own stated preferences followed if they were to lose decision-making capacity. Of the 646 SUPPORT patients, 504 (78.0%) would prefer to have their family and physician decide and 22.0% would prefer to have their advance preferences followed. Independent predictors of preference for family and physician decision-making included not wanting to be resuscitated and having a surrogate decision-maker.

CONCLUSIONS

Most inpatients who are older or have serious illnesses would not want their stated resuscitation preferences followed if they were to lose decision-making capacity. Most patients in both groups would prefer that their family and physician make resuscitation decisions for them. These results underscore the need to understand resuscitation preferences within a broader context of patient values.

Interlaced x-ray microplanar beams: a radiosurgery approach with clinical potential

Studies have shown that x-rays delivered as arrays of parallel microplanar beams (microbeams), 25- to 90-microm thick and spaced 100-300 microm on-center, respectively, spare normal tissues including the central nervous system (CNS) and preferentially damage tumors. However, such thin microbeams can only be produced by synchrotron sources and have other practical limitations to clinical implementation. To approach this problem, we first studied CNS tolerance to much thicker beams. Three of four rats whose spinal cords were exposed transaxially to four 400-Gy, 0.68-mm microbeams, spaced 4 mm, and all four rats irradiated to their brains with large, 170-Gy arrays of such beams spaced 1.36 mm, all observed for 7 months, showed no paralysis or behavioral changes. We then used an interlacing geometry in which two such arrays at a 90-degree angle produced the equivalent of a contiguous beam in the target volume only. By using this approach, we produced 90-, 120-, and 150-Gy 3.4 x 3.4 x 3.4 mm(3) exposures in the rat brain. MRIs performed 6 months later revealed focal damage within the target volume at the 120- and 150-Gy doses but no apparent damage elsewhere at 120 Gy. Monte Carlo calculations indicated a 30-microm dose falloff (80-20%) at the edge of the target, which is much less than the 2- to 5-mm value for conventional radiotherapy and radiosurgery. These findings strongly suggest potential application of interlaced microbeams to treat tumors or to ablate nontumorous abnormalities with minimal damage to surrounding normal tissue.

Distribution of the mammalian Stat gene family in mouse chromosomes

Studies of transcriptional activation by interferons and a variety of cytokines have led to the identification of a family of proteins that serve as signal transducers and activators of transcription, Stats. Here, we report that the seven mouse Stat loci map in three clusters, with each cluster located on a different mouse autosome. The data suggest that the family has arisen via a tandem duplication of the ancestral locus, followed by dispersion of the linked loci to different mouse chromosomes.

Multicopy plasmids are clustered and localized in Escherichia coli

We localized the multicopy plasmid RK2 in Escherichia coli and found that the number of fluorescent foci observed in each cell was substantially less than the copy number of the plasmid, suggesting that many copies of RK2 are grouped into a few multiplasmid clusters. In minimal glucose media, the majority of cells had one or two foci, with a single focus localized near midcell, and two foci near the 1/4 and 3/4 cell positions. The number of foci per cell increased with cell length and with growth rate, and decreased upon entering stationary phase, suggesting a coordination of RK2 replication or segregation with the bacterial cell cycle. Time-lapse microscopy demonstrated that partitioning of RK2 foci is achieved by the splitting of a single focus into two or three smaller foci, which are capable of separating with rapid kinetics. A derivative of the high-copy-number plasmid pUC19 containing the lacO array was also localized by tagging with GFP-LacI. Whereas many of the cells contained numerous, randomly diffusing foci, most cells exhibited one or two plasmid clusters located at midcell or the cell quarter positions. Our results suggest a model in which multicopy plasmids are not always randomly diffusing throughout the cell as previously thought, but can be replicated and partitioned in clusters targeted to specific locations.