No abstract

Strand orientation of transcription in BSC-1 cells infected by simian virus 40 (SV40) was investigated by annealing RNAs extracted from infected cells with asymmetric complementary RNA from SV4O DNA synthesized in vitro by RNA polymerase from Escherichia coli. The results suggest that the early viral RNA sequences (those made before replication of viral DNA) and the late RNA sequences (those synthesized concurrently with replication of viral DNA) are transcribed from opposite strands of the SV40 DNA. The RNA synthesized in vitro is of the same polarity as the early in vivo RNA and is complementary to the late in vivo RNA. Knowledge of strand selection in lytic infection is a step towards understanding the regulation of transcription of this viral DNA.The small DNA virus, simian virus 40 (SV40), grows lytically in permissive African green monkey cells and usually transforms nonpermissive cells, such as mouse or hamster, in which a complete replicative cycle is absent. This difference of interaction with host cells often correlates with a difference in transcription of the viral genome. In the lytic cycle about one-third to one-half of the genome is transcribed early in infection, before replication of viral DNA begins, and comprises the class of early RNA(s) (1-5). After DNA replication has begun, the remainder of the genome (1, 6) is transcribed such that late in infection both early and late RNA sequences are synthesized. In certain transformed cells, transcription is limited to about 40% of the genome (1, 2). Variable amounts of the genome are transcribed in other transformed lines (6).Most of the sequences transcribed in transformed cells are also transcribed early in lytic infection (2, 4).To understand how the various modes of transcription might be regulated, we have investigated which strand of viral DNA serves as template for RNA synthesis in vivo. This was made possible by the observation that, in vitro, Escherichia coli RNA polymerase preferentially transcribes only one DNA strand, often in its entirety (7). We compared viral RNA made early and late in infection with that made in vitro. The results show that early sequences are of the same polarity as the in vitro RNA. whereas the late sequences are complementary to the in vitro RNA. We conclude that early and late viral RNA species are transcribed from different strands of the SV40 DNA. (11)

Abstract. In contrast to observations in bacteria and fungi, frameshift mutations in bacteriophage T4 do not arise during genetic recombination. Nascent mutants, captured in the heterozygous condition, exhibit properties which indicate that the new lesions are located at the extreme tips of the chromosomes, and are segregated by a combination of recombination and replication.Frameshift mutations consist of additions and deletions of small numbers of base pairs,1' 2 and may involve from 1 to at least 20 residues.2' They are efficiently induced and reverted by various acridines,48 and appear first in a heterozygous condition.8' 9The mutagenic acridines intercalate between adjacent base pairs.5' [10][11][12] Mutagenesis was therefore initially imagined to arise from copying errors during DNA replication.13 Inhibition of DNA replication by 5-fluoro-deoxyuridine, however, did not abolish proflavin mutagenesis in bacteriophage T4, although it effectively eliminates base analog mutagenesis.8Acridines are highly mutagenic for bacteriophage T4, but are very weak mutagens for bacteriophage X and for bacteria.', 14 T4 is a rapidly recombining organism,1" whereas X is not.'6 It was therefore suggested that acridines produce frameshift mutations by inducing unequal crossing over. 12 This hypothesis was supported by the discovery of correlations between frameshift mutagenesis and diploidy in the bacterium Escherichia coli,'7 and between frameshift mutagenesis and recombination of outside markers in the yeast Saccharomyces cerevisiae.'8' 19The correlation was less evident in T4, however: newly arisen frameshift mutations appeared to be associated with recombination of outside markers, but these markers were recovered in the heterozygous condition,9 and the heterozygous regions were much longer than typical recombinational overlaps.20' 21 Furthermore, results to be presented here show a striking lack of correlation in T4 between frameshift mutagenesis and recombination of very close outside markers. It should also be emphasized that contemporary theory of the mechanics of recombination22 does not easily encompass a process of unequal crossing over directed by agents which intercalate between adjacent base pairs. Since frameshift mutants frequently revert by intracistronic suppressor mutations,' the revertant polypeptides may contain multiple amino acid substitutions, including deletions and additions. Comparisons of wild-type and re-617

Strand orientation of transcription in BSC-1 cells infected by simian virus 40 (SV40) was investigated by annealing RNAs extracted from infected cells with asymmetric complementary RNA from SV4O DNA synthesized in vitro by RNA polymerase from Escherichia coli. The results suggest that the early viral RNA sequences (those made before replication of viral DNA) and the late RNA sequences (those synthesized concurrently with replication of viral DNA) are transcribed from opposite strands of the SV40 DNA. The RNA synthesized in vitro is of the same polarity as the early in vivo RNA and is complementary to the late in vivo RNA. Knowledge of strand selection in lytic infection is a step towards understanding the regulation of transcription of this viral DNA.The small DNA virus, simian virus 40 (SV40), grows lytically in permissive African green monkey cells and usually transforms nonpermissive cells, such as mouse or hamster, in which a complete replicative cycle is absent. This difference of interaction with host cells often correlates with a difference in transcription of the viral genome. In the lytic cycle about one-third to one-half of the genome is transcribed early in infection, before replication of viral DNA begins, and comprises the class of early RNA(s) (1-5). After DNA replication has begun, the remainder of the genome (1, 6) is transcribed such that late in infection both early and late RNA sequences are synthesized. In certain transformed cells, transcription is limited to about 40% of the genome (1, 2). Variable amounts of the genome are transcribed in other transformed lines (6).Most of the sequences transcribed in transformed cells are also transcribed early in lytic infection (2, 4).To understand how the various modes of transcription might be regulated, we have investigated which strand of viral DNA serves as template for RNA synthesis in vivo. This was made possible by the observation that, in vitro, Escherichia coli RNA polymerase preferentially transcribes only one DNA strand, often in its entirety (7). We compared viral RNA made early and late in infection with that made in vitro. The results show that early sequences are of the same polarity as the in vitro RNA. whereas the late sequences are complementary to the in vitro RNA. We conclude that early and late viral RNA species are transcribed from different strands of the SV40 DNA. (11)