No abstract

No abstract

Cycloheximide (CHX), an inhibitor of cytosolic (80S) protein synthesis in eucaryotes, causes phase shifts of the circadian clock of Neurospora crassa when administered as 4-h pulses to cultures in liquid medium. Differential effects of the pulses at different phases of the circadian cycle were observed and plotted as a phase-response curve (PRC). Nearly all phase shifts observed were phase advances, with maximum sensitivity in the middle of the subjective day. Inhibition of protein synthesis by CHX was the same at both phases of the cycle. The PRC was the same at 20 and 25 degrees C. Dose-response curves for the effects of CHX on phase shifting and inhibition of protein synthesis were determined and showed a striking parallel in the responses of these two phenomena to CHX. These results support the view that synthesis of one or more proteins at specific phases of the circadian cycle is necessary for the normal operation of the circadian clock of Neurospora.

We have characterized at the molecular level seven chromosome-specific libraries constructed in phage λ Charon 21A from flow-sorted human chromosomes. The purity of libraries prepared from chromosomes sorted from hamster × human cells was estimated by species-specific hybridization and ranged from 48% to 83% of clones containing human inserts. Among libraries of chromosomes from human cells, mass screenings were made for repetitive sequences and 20 clones from the # 18 and # 20 libraries were analyzed in detail. Ten to fifteen percent of all clones contain sequences which can be mapped; 80 100% of these derive from the intended chromosome of origin, demonstrating very high purity and a 35 × enrichment of chromosome-specific sequences over a total genomic library. The two libraries contain a high, though dissimilar, percent of repeat-containing clones; the # 18 library has 55% repetitive clones and the # 20 library 85%. This dissimilarity may be due to a difference in insert size distribution, since the # 18 library has smaller inserts than the # 20. This could be caused by variation in extent of digestion of insert DNA and/or differences in sequence organization between the two chromosomes. A method more sensitive than conventional plaque-lift screening was used to detect repetitive inserts; in this way nearly all repetitive clones could be eliminated before purification of their DNAs.

No abstract