We constructed and analyzed a synthetic poly(A) (SPA) site that was based on the highly efficient poly(A) signal of the rabbit I~-globin gene. By use of the SPA, we demonstrate that the minimum sequences required for efficient polyadenylation are the AATAAA sequence and a GT/T-rich sequence with the correct spacing of 22-23 nucleotides between them. When placed downstream of the poly(A) site of the human a2-globin gene, the SPA is used exclusively. We predict that the SPA, with its more extensive GT/T-rich sequence, is a more efficient poly(A) site than c~-globin. Also, we compared the use of the SPA when it is placed either in the exon 3 or intron 2 of the rabbit 13-globin gene. When in the exonic position, SPA is used 10-fold more than the regular poly(A) site of rabbit 13-globin. In contrast, when it is in the intronic location, no detectable use of SPA is observed; however, the deletion of the donor site of intron 2 reactivates the intronic positioned SPA. These results indicate that the splicing of intron 2 in the rabbit 13-globin gene occurs ahead of polyadenylation and have important implications for termination of transcription. Polyadenylation, although required for termination of transcription, is not sufficient; therefore, additional termination signals for RNA polymerase II must exist.

Termination of transcription by RNA polymerase II has been postulated to involve a pausing process. We have identified such a pause signal, 350 bp into the 3′ flanking region of the human alpha 2 globin gene at a position where termination is thought to occur. We show that this pause signal enhances the utilization of an upstream poly(A) site which is otherwise out‐competed by a stronger downstream poly(A) site. We also demonstrate that the pause site rescues a poly(A) site that is inactive due to its location within an intron. Using nuclear run‐on analysis we show that elongating RNA polymerase II molecules accumulate over this pause signal. Furthermore we show that when the pause site is positioned immediately downstream of a strong poly(A) signal, significant levels of transcription termination take place.

Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.

Previous studies [1,2] suggested but did not prove that the sea urchin H2A histone gene possesses strong transcriptional termination signals close to, but separate from, the 3' processing signals. In this study we have demonstrated by two independent approaches that these sequences elicit authentic transcriptional termination. First we show by nuclear run off analysis that nascent transcription terminates in the immediate 3' flanking region of the H2A gene, in an A-rich region. Second we show that these termination signals prevent transcriptional read through when placed in the intron of a globin gene. The intronic position of the termination signal rules out any effect on steady state mRNA levels. We have therefore defined DNA sequences which act as a transcription terminator when placed in heterologous RNA polymerase II genes.

No abstract