Circoviruses are a diverse group of animal and plant pathogens with undefined relationships to one another but for their non-geminate, non-enveloped capsids and circular, single-stranded DNA genomes. The sequences of the beak and feather disease virus and porcine circovirus genomic DNAs are presented and analyzed in the context of the other members of the family. Sequence comparisons, inferred phylogenies, and geographic occurrence suggest that the ambisense circoviruses, particularly the beak and feather disease virus, represent an evolutionary link between the geminiviruses and the plant circoviruses. We propose that the family members be reclassified into three groups: The family Circoviridae consists of the animal pathogens (beak and feather disease virus and porcine circovirus) that possess ambisense genomes with striking similarities to the geminiviruses. The BBTV-like viruses include the plant pathogens (coconut foliar decay virus, banana bunchy top virus, subterranean clover stunt virus) with a geminivirus-like stem-loop element in their DNAs, and single to multiple component genomes. The chicken anemia virus is an unassigned virus possessing unique characteristics bearing little similarity to the other ssDNA viruses.

The relationship of valine resistance .to the expression of the ilvGEDA operon of Escherichia cob K-12 has been determined. DNA sequence and in vivo protein analyses indicate that in wild-type E. coli K-12 there is a frameshift site within the gene (ilvG) for valine resistance. The ilvG+2096 (formerly designated ilv02096) mutation displaces this frameshift site, resulting in the~expression of ilvG and the relief of transcriptional-polarity on the distal genes of this operon. Thus, the "ilvO" mutation, which concomitantly confers valine resistance and increased expression of the ilvEDA genes, is, in fact, the "reversion" of a polar site within the first structural gene-of the.ilvGEDA operon.

The regulatory region for the ivGEDA operon of Escherichia coli K-12 has been located and characterized. ilv leader RNA transcribed from this region is described, and the DNA sequence of the region is presented. This DNA sequence contains a transcription promoter, a region coding for a 32-amino-acid polypeptide containing multiple isoleucine, valine, and leucine codons, and a transcription termination site preceding the first structural gene. The mutually exclusive secondary structures of the leader RNA have been analyzed. On the basis of these data, a model for the multivalent attenuation of the ilvGEDA operon is proposed.The ilvGEDA genes of Escherichia col' K-12 are multivalently regulated by the percent aminoacylation of tRNAVal, tRNAIle, and tRNALeu (1, 2), and strains containing altered Rho factor exhibit increased expression of this operon (3,4). It has, therefore, been suggested that this operon might be regulated by an attenuator mechanism (3-5) similar to that proposed for several other amino acid biosynthetic operons (6-11). We have now defined the location of the regulatory region for the ilvGEDA operon as being promoter proximal to the livG gene. We show that a previously described short RNA transcript is an ilv leader RNA which is transcribed from the regulatory region. We designate the probable sites of transcriptional initiation and termination of this leader RNA by in vitro transcription. We also report here the DNA sequence of the IlvGEDA promoter-attenuator region and show that it contains multiple codons for isoleucine, valine, and leucine within a potential coding region for a short leader polypeptide. On the basis of these results and by analysis of the possible secondary structures of the ilv leader RNA, we conclude that this operon is regulated by translational control of transcription termination at an attenuator site (12, 13). MATERIALS AND METHODSRestriction enzymes were obtained from New England Biolabs, were prepared by standard methods, or were a gift of Charles Yanofsky. E. coli DNA-dependent RNA polymerase was obtained from New England Biolabs. Plasmid pVH153, containing the regulatory-region of the trp operonwith a G-to-A transition at base pair 130 of the attenuator (14), was a generous gift of G. Zurawski and C. Yanofsky. The plasmid, pRL5, was constructed and DNA was prepared as described (15).In vitro RNA transcription was performed as described (6,15) with the addition of heparin as described by Majors (16). The DNA sequence analysis was performed as described by Maxam and Gilbert (17). RESULTSThe in vitro transcription of two plasmids that contain the regulatory region for the ilvGEDA operon. yields two short leaderlike RNAs 180 and 250 nucleotides long (see Fig. 4, lane 1; ref. 15). This in vitro labeled RNA was hybridized to a Southern blot (18) of a restriction endonuclease digest of the regulatory region. Both transcripts hybridized to the 300-base-pair Hae III/HinfI fragment (see Fig. 2) located 1.8 kilobase pairs in front of the DNA sequence encoding the ilv...

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Saccharomyces cerevisiae can degrade allantoin in five steps to glyoxylate, ammonia, and "CO2."We previously demonstrated that synthesis of the urea carboxylase-allophanate hydrolase multienzyme complex is contingent upon the presence of allophanic acid, the product of the urea carboxylase reaction. Since these enzymes catalyze the last two reactions of allantoin degradation, experiments were performed to establish whether or not the presence of allophanic acid was required for synthesis of any other enzymes participating in this degradative pathway. The data presented here indicate that allophanic acid is required for synthesis of all enzymes participating in allantoin degradation. This conclusion is based upon the observation that: (i) wild-type strains produced a large amount of allantoinase upon addition of allantoin, allantoate, ureidoglycolate, or urea to the medium, (ii) no increase in activity was observed unless the added compound could be metabolized to allophanate, (iii) strains lacking allophanate hydrolase contained large amounts of allantoinase even in the absence of added urea, and (iv) the urea analogue, formamide, was capable of inducing allantoinase synthesis in wild-type strains but would not serve this function in a strain lacking urea carboxylase.