Expression of MXI1, a Myc antagonist, is regulated by Sp1 and AP2.

MXI1, a member of the MAD family of Myc antagonists, encodes a transcription factor whose expression must be tightly regulated to maintain normal cell growth and differentiation. To more closely investigate the transcriptional regulation of the human MXI1 gene, we have cloned and characterized the MXI1 promoter. After clarification of the 5'- and 3'-untranslated regions of the cDNA (indicating that the true length of the MXI1 transcript is 2643 base pairs), we identified two transcription initiation sites. We subsequently isolated the MXI1 promoter, which is GC-rich and lacks a TATA box. Although it contains at least six potential initiator sequences, functional studies indicate the proximal two initiator sequences in combination with nearby Sp1 and MED-1 sites together account for virtually all promoter activity. We also demonstrate that MXI1 promoter activity is repressed by high levels of AP2. These studies provide further insight into the complex regulatory mechanisms governing MXI1 gene expression and its role in cellular differentiation and tumor suppression.

Application of optimized parallel processing digital computers and numerical approximation methods to the ultra high-speed three-dimensional reconstruction of the intact thorax.

In order to achieve the computational capability to carry out many thousands of cross-sectional reconstructions, necessary to support a prototype high temporal and spatial resolution cylindrical scanning multiaxial tomographic unit, a series of design, software simulation, and fabrication studies is underway to develop a special-purpose high-speed reconstruction computer. This processor will rely upon integrated circuit arithmetic components of advanced design, and highly parallel architecture to execute X-ray based transaxial reconstruction algorithms at the rate of hundreds of cross sections/sec.