A transcription fork model for Pol IV and Pol V-dependent RNA-directed DNA methylation.

In Arabidopsis thaliana, nuclear multisubunit RNA polymerase IV (Pol IV) and RNA-DEPENDENT RNA POLYMERASE 2 (RDR2) are required for the biogenesis of 24-nucleotide small interfering RNAs (siRNAs) that direct DNA methylation and transcriptional silencing at target loci transcribed by nuclear multisubunit RNA polymerase V (Pol V). Pol IV and RDR2 physically associate and RDR2's polymerase activity in vitro is dependent on Pol IV. RDR2 transcription of nascent Pol IV transcripts might result in discontinuous second strands, analogous to lagging-strand Okazaki fragments generated during DNA replication. In vitro, Pol V is unable to displace nontemplate DNA during transcriptional elongation. This suggests a need for DNA duplex unwinding by helper proteins, perhaps analogous to the helicase-mediated duplex unwinding that occurs at replication forks to enable leading strand synthesis by DNA polymerase ε. A multiprotein complex (DRD1, DMS3, DMS11, RDM1) known to enable Pol V transcription might facilitate duplex unwinding via ATP-dependent DNA translocase, single-stranded DNA binding, and cohesin-like strand capture activities. These considerations are discussed and incorporated into a "transcription fork" model for Pol IV and Pol V-dependent RNA-directed DNA methylation.

MRI and CT of sellar and parasellar disorders.

Discussion of the anatomy of the parasellar region focuses on the optic chiasm and its relationship to surrounding structures. CT and MRI are valuable tools in the imaging of these regions. This article provides a brief review of the technique of CT and MRI with visualized anatomic structures of these areas. A closer look at how CT and MRI are used to delineate the different pathologic processes of the sellar and parasellar regions then follows.

Optic atrophy following jejunoileal bypass.

A 38-year-old woman presented with a bilateral, chronic, progressive optic neuropathy following jejunoileal bypass surgery for morbid obesity. Standardized A-scan orbital echography revealed grossly swollen optic nerve sheaths with a positive 30 degrees test, indicating increased subarachnoid fluid. Review of reported complications of jejunoileal bypass surgery revealed no similar case of what might be termed the "optic nerve ascites syndrome."

Oculomotor nuclear complex infarction. Clinical and radiological correlation.

We describe a patient with an isolated mesencephalic dorsal tegmental infarct affecting the oculomotor nuclear complex and medial longitudinal fasciculus, documented by high-resolution computed tomography, after undergoing percutaneous transluminal coronary artery angioplasty. Clinically, the patient exhibited bilateral ptosis, bilateral internuclear ophthalmoplegia, transient convergence retractory nystagmus, and minimal somnolence. We believe the combined clinical and radiological findings favor the presence of a caudal, dorsal, and paramedian embolic infarct in the territory of the paramedian branches of the mesencephalic artery as the most likely mechanism for these exceptional findings and correlate them with Warwick's scheme of the oculomotor subnuclei.

Effects of swimming pool water on the cornea.

Eye examinations performed on 50 subjects immediately before and after swimming in a chlorinated pool showed that 34 subjects (68%) saw rainbows and/or halos around lights after swimming, a symptom indicating the presence of corneal edema. Forty-seven subjects (94%) had corneal epithelial erosions in a punctate or linear pattern demonstrated by fluorescein staining on slit-lamp examination. No subject experienced a measurable decrease in visual acuity.

[Simultaneous measurement of blood concentrations of O2 and CO with a Van Slyke apparatus]. / Mesures simultanées des concentrations sanguines en O2 et en CO à l'appareil de Van Slyke.

imultaneous analysis of a blood sample for its O2 and CO contents (Co2 and Cco) with the original manometric technique of Van Slyke entails a directional error on Co2 and Cco when blood contains CO. This important drawback is due to the fact that Na2S2O4 employed for the O2 absorption also reduces methemoglobin into hemoglobin which then reabsorbs a variable amount of the extracted CO. This amount is equal to the directional error and results in an overestimation of Co2 and in an underestimation of Cco. In order to analyze and possibly correct for this error, we have measured it by gas chromatography by determining Cco on blood samples with and without O2 absorption by Na2S2O4. The error has been found to be a function of both blood CO and Hb contents. An equation and a nomogram are presented allowing correction of the results obtained by the Van Slyke method. So corrected the results are equally precise in presence and in absence of CO (+/- 0.1 volume %) which makes simultaneous Co2 and Cco measurements with this classical and unexpensive method quite acceptable.