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The pMGA multigene family encodes variant copies of the cell surface haemagglutinin of Mycoplasma gallisepticum. Quantitative Southern blotting, using an oligonucleotide probe complementary to a region conserved in the leader sequence of all known pMGA genes, was used to estimate the number of members of the family in the genome of seven strains of M. gallisepticum. The number of copies estimated to be present in the genome varied from 32 in strain F to 70 in strain R, indicating that the pMGA gene family may be second in size only to the tRNA family among prokaryotes. If all members of the pMGA family are of similar length to those which have been characterized, a minimum of 79 kb (7.7%) of the genome of strain S6, 82 kb (8.2%) of PG31 and 168 kb (16%) of the genome of strain R is dedicated to encoding variants of the same haemagglutinin. The GAA repeat motif identified in the intergenic region between all characterized pMGA genes appeared to be a feature common to most, if not all, pMGA genes, and furthermore probably exclusive to them. The genomic locations of members of the pMGA family were determined by PFGE and Southern blot hybridization of M. gallisepticum strain S6. The hybridizing regions were localized to four separate regions on the chromosome. The pMGA genes are likely to be predominantly arranged as tandem repeats within these regions, similar to the restricted regions for which the genomic sequence has been determined.

The promoter, operator, and 5' and 3' ends of the mRNA of the Escherichia coli gene aroG (encoding the phenylalanine-sensitive 3-deoxy-arabinoheptulosonate-7-phosphate synthase) were located. Primer extension analysis and nuclease Si mapping of in vivo transcripts were used to determine the 5' and 3' ends, respectively, of the mRNA. Both ends exhibited some heterogeneity with respect to length. The 3' end of the major molecular species was located within a region that has structural homology with known rho-independent terminators. The location of the aroG promoter was identified in both strands of the DNA by in vitro DNase I footprinting and methylation protection experiments with RNA polymerase. In these experiments, a region of up to 80 base pairs (bp) was protected by the binding of RNA polymerase. The location of the aroG operator was also identified in both strands of the DNA by in vitro DNase I footprinting with pure TyrR. TyrR protected 26 to 28 bp of DNA containing a 22-bp palindrome (TYR R box) and overlapping the'-35 region of the promoter. Mutations in the aroG regulatory DNA were isolated by site-directed mutagenesis and cloned in a low-copy-number plasmid to generate aroG-lac fusions. The effects of the mutations on the regulation of aroG expression were determined by measuring the (-galactosidase activities of the fusions in strains with tyrR, tyrR+, and multicopy tyrR' genotypes. The results of this mutant analysis confirmed that the aroG operator contains a single TYR R box.In Escherichia coli, the phenylalanine-sensitive 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase (EC 4.1.2.15) is one of three isoenzymes that catalyze the first committed step in the biosynthesis of tyrosine, phenylalanine, tryptophan, and other aromatic compounds (22,36). This enzyme is encoded by the gene aroG. The expression of aroG and a number of other genes that encode enzymes and proteins required for the biosynthesis of these compounds and the uptake of the three aromatic amino acids is regulated by TyrR, the protein product of the gene tyrR (15). The eight operons that TyrR is known to regulate constitute the TyrR regulon (36). The transcription of aroG is repressed by TyrR (26) in the presence of excess phenylalanine and tryptophan (6, 7). Other operons in the TyrR regulon are repressed by TyrR in the presence of tyrosine alone or combinations of tyrosine and tryptophan or phenylalanine; the degree of maximal repression varies widely within the regulon (for a review, see reference 36). The molecular basis of the interaction of TyrR with its target operons is of interest, since the situation in which one regulatory protein exerts differential control over a number of different target genes appears to be common in both procaryotes and eucaryotes.In previous studies, we determined the nucleotide sequence of aroG and surrounding DNA (17) and used a promoter-cloning vector to localize the aroG promoter and operator within a 276-base-pair' (bp) Sau3AI fragment (18). The investigations that we describe in this communic...

We analyzed the segment of DNA which contains the expressed pMGA gene from one strain of Mycoplasma gallisepticum in normal (strain S6) cells and in cells in which pMGA1.1 gene expression had ceased as a consequence of in vitro culture in the presence of pMGA1.1-specific antibodies. Sequence analysis of isolates lacking pMGA1.1 expression revealed that this gene, which is typically expressed, exhibited sequence changes within a region 5′ to its promoter. Specifically, pMGA1.1+ cells contained a (GAA)12 motif upstream of the promoter, whereas in pMGA1.1− cells the corresponding region contained a (GAA)10 motif; when such cells were grown in medium no longer containing pMGA-specific antibodies, pMGA1.1 was reexpressed and the 5′ (GAA)12 motif was restored. Two other genes, pMGA1.9 and pMGA1.2, were also shown to acquire a (GAA)12 motif in clones which expressed these genes. The results imply the evolution by the pMGA genes of M. gallisepticum of a novel transcriptional requirement which facilitates rapid and reversible switches in the pMGA expression pattern.

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Pulsed field gel electrophoresis of restriction endonuclease digested genomic DNA from a collection of clinical isolates of Rhodococcus equi was used to compare strain diversity on different Thoroughbred horse farms over time. Restricted diversity was found among the isolates tested, as the same strains were detected on multiple farms and in multiple years. Marked variation occurred in strain prevalence with some strains being represented by single isolates, and the most prevalent by 26 isolates. There were dominant strains on some farms and the prevalence of some strains differed between farms. Infection with multiple strains was noted in some cases where multiple isolates from a single foal were examined.

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