Tl Site Research Articles

NMR shifts and relaxation of 205Tl in the Tl 2Ba 2CaCu 2O 8 (2212) high-T c-superconductor

We report on 205Tl NMR shifts and relaxation in the normal and in the superconducting state. The NMR spectra show two Tl sites. Comparison with reference compounds suggests Tl 3+ valency in the Tl-O-planes. This site shows no Knight shift taking demagnetization into account. A defect line is attributed to Tl at a Ca site. This line shows appreciable Knight shift. In the superconducting state a strong line broadening in the NMR line is visible. The temperature dependence of the second moment is discussed.

Anomalous Nuclear Relaxation and Knight Shift Behaviors of205Tl in High-TcTl2Ba2Ca1Cu2O8+δ

The nuclear relaxation rate, 1/ T 1 , and Knight shift have been investigated between 4.2 K and 300 K by 205 Tl nuclear magnetic resonance (NMR) for Tl 2 Ba 2 Ca 1 Cu 2 O 8+δ with T c =100 K. It has been observed that there are two Tl sites possessing a different electronic state. The distinct decrease of 1/ T 1 and Knight shift for both Tl sites has been found just below an onset temperature of T 0 =115 K rather than a zero-resistance temperature of T c =100 K. Above T 0 , 1/ T 1 is not of the Korringa-type with T 1 T =const., but exhibits weak temperature dependence, resembling the behavior of the CuO 2 plane site of YBa 2 Cu 3 O 7 . The 1/ T 1 for Tl sites possessing no magnetic moments is demonstrated to reflect the strong spin correlation above T 0 through the hybridization with the Cu d-wave function.

Surface mapping of mouse thymocytes.

The blocking method used previously for determining the relative positions of different components of the cell surface was modified by first fixing the cells with paraformaldehyde. This technique was applied to the H-2K (K), H-2D (D), TL, Lyt-1, and Lyt-2 surface components of mouse thymocytes, and the results were compared in parallel with data obtained with the original technique with unfixed cells. Previous mapping data with unfixed cells, indicating the positions of these molecules relative to one another, were confirmed with paraformaldehyde-fixed cells, with one exception. On unfixed cells, D and TL appeared sufficiently adjacent to produce mutual interference in the attachment of anti-D and anti-TL antibodies. With paraformaldehyde-fixed cells this was not so, D and TL appearing sufficiently separated from one another to obviate interference in the attachment of anti-D and anti-TL antibodies. The previously reported close association of K with Lyt-1 and of D with Lyt-2 were demonstrable equally with unfixed and paraformaldehyde-fixed thymocytes. It is suggested that activation of D sites, and alternatively of TL sites, by antibody in the present experiments brings these two molecules into apposition and that this movement may exemplify a mechanism concerned in immunological recognition and response.

PL-promoted transcription of the promoter-proximal N-trp region is insensitive to chloramphenicol in the absence of N function.

When the trp operon is translocated into the early region of lambda phage, transcription originated at the PL promotor is known to be modified by function of the N gene product so that transcription of the operon continues when translation is blocked by nonsense mutations or by ribosomal antibiotics. When N function is deficient in a phage that joins the trp operon to a point distal to the N gene, deleting the tL site, nonsense mutations (Franklin, 1974) or chloramphenicol (Nakamura et al., accompanying paper) again block transcription of the bacterial operon. However, here we report that transcription over about the first 800 nucleotide pairs starting from the PL promotor of the N-trp operon is still insensitive to chloramphenicol even in the absence of N function. The region covers the full N gene and the initial bit of the trp operon.

Restoration of polarity by N-deficiency in lambda phage containing a translocated trp operon segment

When translation of trp mRNA is terminated by a nonsense codon or by antibiotics like chloramphenicol, the amount of the mRNA distal to the blocked ribosomes is found at much lower levels ("polarity"). Polarity is alleviated when the trp mRNA is formed as part of a long transcript from the phage lambda promoter PL (Segawa and Imamoto, 1974; Franklin, 1974); but the relief of polarity is itself largely dependent on the lambda protein N. In a phage that joins the trp operon segment (trpD, C, B A) to a point distal to the N gene, lacking the tL site, synthesis of trp mRNA starting at the PL promoter continues even when translation is generally inhibited by chloramphenicol, but in the absence of functional N gene product synthesis of the mRNA can be blocked by the antibiotic. Unexpectedly, in the absence of N function, even when translation is occurring, weak termination of transcription occurs at some sites in the translocated trp operon.

Altered reading of genetic signals fused to the N operon of bacteriophage λ: Genetic evidence for modification of polymerase by the protein product of the N gene

Nonsense mutations, which are strongly polar when transcribed in the context of Escherichia coli, are no longer polar when the trp genes that carry them are fused to the N operon of bacteriophage λ. This relief of polarity depends on specific features of the phage operon: initiation from the phage pL promoter and the presence of a functional N protein. The effect is probably analogous to the anti-transcription-termination effected by N protein at the tL site in normal λ, but tL itself is not required. Under conditions of limiting N protein, total transcription is reduced, but the proportion of polarity relief remains high. It is concluded that transcription initiated at pL by E. coli RNA polymerase is so modified by interaction with N protein that even distant nonsense codons are no longer read as a cause of polarity.