A modification of the supraorbital keyhole approach, the eyebrow incision-minisupraorbital craniotomy with orbital osteotomy, is described. Unique to this approach is a one-piece supraorbital craniotomy, measuring 2.5 x 3.5 cm, that incorporates the orbital rim and roof and the frontal process of the zygomatic bone through an eyebrow incision. The orbital osteotomy facilitates view of the anterior and middle cranial fossa through the operating microscope, as well as the maneuverability of instruments through a small craniotomy. A pericranial flap is elevated with its base at the orbit and used for closure of the frontal sinus, if necessary. The approach was used successfully in elective surgery of 10 aneurysms of the anterior circulation. The mean aneurysm size was 5.9 mm, with a range of 4 to 10 mm. Advantages of this approach include minimal disruption and exposure of normal brain tissue, reduced frontal lobe retraction, and an excellent postoperative cosmetic result. The approach is performed quickly by virtue of a limited skin incision with minimal temporalis muscle dissection and a small bone flap. The neuroendoscope, although helpful at times, is not essential and no special instruments or intraoperative image guidance is required. Relative contraindications include the presence of a large frontal sinus, severe brain edema, and recent subarachnoid hemorrhage. In addition, this approach has not been used for the treatment of giant intracranial aneurysms.

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An interesting feature of trypanosome genome organization involves genes transcribed by RNA polymerase III. The U6 small nuclear RNA (snRNA), U-snRNA B (the U3 snRNA homolog), and 7SL RNA genes are closely linked with different, divergently oriented tRNA genes. To test the hypothesis that this association is of functional significance, we generated deletion and block substitution mutants of all three small RNA genes and monitored their effects by transient expression in cultured insect-form cells of Trypanosoma brucei. In each case, two extragenic regulatory elements were mapped to the A and B boxes of the respective companion tRNA gene. In addition, the tRNAThr gene, which is upstream of the U6 snRNA gene, was shown by two diferent tests to be expressed in T. brucei cells, thus confirming its identity as a gene. This association between tRNA and small RNA genes appears to be a general phenomenon in the family Trypanosomatidae, since it is also observed at the U6 snRNA loci in Leishmania pifanoi and Crithidiafasciculata and at the 7SL RNA locus in L. pifanoi. We propose that the A-and B-box elements of small RNA-associated tRNA genes serve a dual role as intragenic promoter elements for the respective tRNA genes and as extragenic regulatory elements for the linked small RNA genes. The possible role of tRNA genes in regulating small RNA gene transcription is discussed.Protozoa of the family Trypanosomatidae include a diverse number of organisms, several of which are parasites of great medical and economical importance. Interest in these cells stems not only from their parasitic lifestyles but also from evolutionary considerations which place the trypanosomatids among the deepest branches of the eukaryotic lineage (23). In recent years, the analysis of trypanosome gene expression has been quite rewarding and has provided evidence for novel mechanisms, including RNA editing, trans splicing, and polycistronic transcription units. Notwithstanding many advances in our understanding of trypanosome biology, our knowledge of promoter structures in these organisms is still limited to a few examples. These include three RNA polymerase (pol) I promoters of the procyclic acidic repetitive protein (PARP) (1, 3), variant surface glycoprotein (27,28), and rRNA (27) genes and one pol III promoter of the trans-spliceosomal U2 small nuclear RNA (snRNA) gene (4). For the protein-coding genes, it has become evident that the eukaryotic rule of one promoter for one gene does not apply. Rather, several protein-coding genes are transcribed as part of complex polycistronic transcription units. This functional economy in terms of usage of promoter signals is accompanied by a high density of genetic information in the nuclear genome, with protein-coding genes being separated only by a hundred to a few hundred nucleotides.Another interesting feature of trypanosome genome organization was noted for some of the genes transcribed by pol III (4, 19). In Trypanosoma brucei, the genes coding for the trans-spliceosomal U6 snRNA, the U3 snRNA homol...

Transcription of U2 small nuclear RNA (snRNA) genes in eukaryotes is executed by RNA polymerase II and is dependent on extragenic cis-acting regulatory sequences which are not found in other genes. Here we have mapped promoter elements of the Trypanosoma brucei U2 snRNA gene by transient DNA expression of mutant constructs in insect form trypanosomes. Unlike other eukaryotic U2 snRNA genes, the T. brucei homolog is transcribed by an RNA polymerase III-like enzyme on the basis of its sensitivity to the inhibitors ot-amanitin and tagetitoxin. Thus, the trypanosome U2 snRNA provides a unique example of an RNA polymerase III transcript carrying a trimethylated cap structure. The promoter of this gene consists of three distinct elements: an intragenic sequence close to the 5' end of the coding region, which is probably required to position the polymerase at the correct transcription start site; and two extragenic elements, located 110 and 160 nucleotides upstream, which are essential for U2 snRNA gene expression. These two elements closely resemble both in sequence and in distance from each other the A and B box consensus sequences of the internal control regions of tRNA genes. (203) 785-3864. t Permanent address: Department of Human Biopathology, University of Rome, La Sapienza, 00100 Rome, Italy.into the 5'-flanking region of the U2 snRNA gene, the specificity of the U2 promoter switches from RNA polymerase II to RNA polymerase III.Promoter elements of invertebrate U-snRNA genes have not yet been analyzed in such detail. Expression of the sea urchin U1 snRNA gene requires a DSE located between -318 and -300 and a PSE centered at -55 (41). On the other hand, the sea urchin U2 snRNA gene contains four cis-acting elements, including a TATA box at -25 and a PSE at -55 (30). Since there is no similarity between the regulatory elements required for expression of the sea urchin Ul and U2 snRNA genes, it appears that each sea urchin U-snRNA gene utilizes distinct promoter elements.In contrast, in higher plants the promoter elements for the RNA polymerase II-and RNA polymerase III-transcribed U-snRNA genes are identical (38,40 (27,42). Immunoprecipitations with antibodies against the trimethylated cap structure of U-snRNAs have identified a number of possible candidates for U-snRNAs in trypanosomes (25,33). Of these, four have been characterized in more detail, and on the basis of structural homology with other eukaryotic U-snRNAs, three have been identified as U2, U4, and U6 snRNAs (25,32,33

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