Extensive movement of LINES ONE sequences in beta-globin loci of Mus caroli and Mus domesticus.

LINES ONE (L1) is a family of movable DNA sequences found in mammals. To measure the rate of their movement, we have compared the positions of L1 elements within homologous genetic loci that are separated by known divergence times. Two models that predict different outcomes of this analysis have been proposed for the behavior of L1 sequences. (i) Previous theoretical studies of concerted evolution in L1 have indicated that the majority of the 100,000 extant L1 elements may have inserted as recently as within the last 3 million years. (ii) Gene conversion has been proposed as an alternative to a history of prolific recent insertions. To distinguish between these two models, we cloned and characterized two embryonic beta-globin haplotypes from Mus caroli and compared them with those of M. domesticus. In 9 of 10 instances, we observed an L1 element to be present in one chromosome and absent at the same site in a homologous chromosome. This frequency is quantitatively consistent with the known rate of concerted evolution. Therefore, we conclude that gene conversion is not required for concerted evolution of the L1 family in the mouse. Furthermore, we show that the extensive movement of L1 sequences contributes to restriction fragment length polymorphism. L1 insertions may be the predominant cause of restriction fragment length polymorphisms in closely related haplotypes.

Replication of human cytomegalovirus at supra-optimal temperatures is dependent on the virus strain, multiplicity of infection and phase of virus replication.

The kinetics of replication of five strains of human cytomegalovirus (CMV) were studied to determine the influence of (i) temperature, (ii) virus strain, (iii) m.o.i. and (iv) cell type. Relative to growth at 37 degrees C (m.o.i. = 3 to 9) eclipse periods were extended from 24 to 48 h at 33 degrees C and to 72 h at 40.5 degrees C. Yields were reduced at 33 degrees C and almost eliminated at 40.5 degrees C. No replication occurred in most instances at 40.5 degrees C and with 0.05 p.f.u./cell. Temperature shift studies (40.5 to 37 degrees C) indicated that the block to replication at 40.5 degrees occurred about 12 to 16 h p.i. resulting in little synthesis of CMV DNA or late antigens. The degree of inhibition of late functions at 40.5 degrees C is virus strain and m.o.i. dependent, but is not dependent on the type of fibroblastic cell used. These data suggest that persistent CMV infections are favoured at 40.5 degrees C.

Pleiotropic effects of mutants in gene A of bacteriophage phi chi 174.

It has previously been established that the functional gene A product of phi chi X 174 is required for double-stranded DNA replication and that mutants in gene A affect the lysis of the host cell. We report here other alterations of normal phenotype for a subset of gene A mutants suggesting additional functions of gene A. Mutants in the subset failed to terminate cellular DNA synthesis and were unable to efficiently inactivate the colony-forming ability of the host. Two mutants in a second group retained the ability to kill the infected cell, although only one of these mutants efficiently terminated cellular DNA synthesis. Normal termination of cellular DNA synthesis did not occur by the production of random multiple breaks in the DNA, although it may have occurred by the selective production of breaks in newly synthesized DNA. It has previously been shown that two protein products are produced from the gene A region, the smaller of which is a C-terminal fragment of the larger. The separate phenotypes reported here for the two groups of mutants in gene A are consistent with separate functions for the two gene products previously reported.

Stimulation of cellular DNA synthesis by human cytomegalovirus.

Human cytomegalovirus (CMV) is able to induce cellular DNA synthesis in both permissive (human embryonic lung) and nonpermissive (Vero) cells. The induction of cell DNA synthesis was assayed by the incorporation of [methyl-(3)H]thymidine into macromolecules having the buoyant density characteristics of cell DNA. The DNA synthesis induced by CMV infection appears to represent normal semiconservative replication as opposed to repair synthesis. Both strains of CMV tested were capable of inducing cell DNA synthesis. Virus exposed to heat or UV light prior to infection lost the ability to induce DNA synthesis, indicating that a virus-coded function expressed after infection is responsible for stimulation of cell DNA synthesis.

Process of infection with bacteriophage phiX174. XXXV. Cistron 8.

Twenty-two new amber and ochre mutants of phiX174 were isolated and classified into complementation groups. Three ochre mutants gave positive complementation tests with reference mutants in the seven previously defined groups and thus represent an eighth cistron. Studies of the physiology of infection in the nonpermissive condition for mutants in cistron VIII yielded the following information. (i) Replicative-form synthesis proceeds at a normal rate, and is turned off at the usual time. (ii) Synthesis of single-stranded deoxyribonucleic acid (DNA) is delayed until nearly 40 min after infection (in the absence of lysis), at which time a slow synthesis of infectious phage particles commences. The synthesis of infectious particles at late times is interpreted as a consequence of "leakage," and indicates that the cistron VIII product is required in very small quantities. (iii) During the normal period of single-strand synthesis, most of the replicative-form DNA is found in a form with properties similar to those of the transient intermediates of single-strand DNA synthesized during normal infection.