Genetic control of RNA polymerase I-stimulated recombination in yeast.

We examined the genetic control of the activity of HOT1, a cis-acting recombination-stimulatory sequence of Saccharomyces cerevisiae. Mutations in RAD1 and RAD52 decrease the ability of HOT1 to stimulate intrachromosomal recombination while mutations in RAD4 and RAD50 do not affect HOT1 activity. In rad1 delta strains, the stimulation of excisive recombination by HOT1 is decreased while the rate of gene replacement is not affected. In rad52-8 strains the ability of HOT1 to stimulate both excisive recombination and gene replacement is decreased. All of the recombinants in the rad52-8 strains that would be categorized as gene replacements based on their phenotype are diploids apparently derived by endomitosis and excisive recombination. Studies on rad1 delta rad52-8 strains show that these mutations interact synergistically in the presence or absence of HOT1, resulting in low levels of recombination. The rate of gene replacement but not excisive recombination is stimulated by HOT1 in rad1 delta rad52-8 strains. Taken together, the results show that HOT1 stimulates exchange using multiple recombination pathways. Some of the activity of HOT1 is RAD1-dependent, some is RAD52-dependent, and some requires either RAD1 or RAD52 as suggested by the synergistic interaction found in double mutant strains. There is also a component of HOT1 activity that is independent of both RAD1 and RAD52.

Serologic methods for detection of Pasteurella multocida infections in nasal culture negative rabbits.

An agar gel-diffusion test (AGDT) and an enzyme-linked immunosorbent assay (ELISA) were utilized to detect serum antibodies against Pasteurella multocida in naturally infected rabbits derived originally from a Pasteurella-free colony. The antigen used in both assays was purified from a serotype 3 (P-1059) strain of P. multocida. Among 47 serum samples tested 15 (32%) were seropositive; 12 (26%) of which were both AGDT and ELISA-positive, while 3 (6%) were ELISA-positive only. All rabbits examined were normal clinically and negative to repeated nasal cultures, but subsequent cultures at necropsy demonstrated the presence of P. multocida in 11 of the AGDT-positive rabbits and in 14 of the ELISA-positive rabbits. The organism was isolated most frequently from the naso-oropharynx and the tympanic bullae. Serotyping of isolates recovered from the nasopharynx were determined to be serotype 3 or 3,12. Ten seronegative rabbits also were necropsied and none were found harboring P. multocida. These preliminary data indicate that the application of an enzyme-linked immunosorbent assay may prove efficacious in identifying apparently healthy, consistently nasal culture-negative rabbits as subclinical carriers of P. multocida.