Physical map location of the Escherichia coli gene encoding phosphoglycerol transferase I

Cloning, sequencing, and characterization of the cgmB gene of Sinorhizobium meliloti involved in cyclic beta-glucan biosynthesis

Periplasmic cyclic beta-glucans of Rhizobium species provide important functions during plant infection and hypo-osmotic adaptation. In Sinorhizobium meliloti (also known as Rhizobium meliloti), these molecules are highly modified with phosphoglycerol and succinyl substituents. We have previously identified an S. meliloti Tn5 insertion mutant, S9, which is specifically impaired in its ability to transfer phosphoglycerol substituents to the cyclic beta-glucan backbone (M. W. Breedveld, J. A. Hadley, and K. J. Miller, J. Bacteriol. 177:6346-6351, 1995). In the present study, we have cloned, sequenced, and characterized this mutation at the molecular level. By using the Tn5 flanking sequences (amplified by inverse PCR) as a probe, an S. meliloti genomic library was screened, and two overlapping cosmid clones which functionally complement S9 were isolated. A 3.1-kb HindIII-EcoRI fragment found in both cosmids was shown to fully complement mutant S9. Furthermore, when a plasmid containing this 3.1-kb fragment was used to transform Rhizobium leguminosarum bv. trifolii TA-1JH, a strain which normally synthesizes only neutral cyclic beta-glucans, anionic glucans containing phosphoglycerol substituents were produced, consistent with the functional expression of an S. meliloti phosphoglycerol transferase gene. Sequence analysis revealed the presence of two major, overlapping open reading frames within the 3.1-kb fragment. Primer extension analysis revealed that one of these open reading frames, ORF1, was transcribed and its transcription was osmotically regulated. This novel locus of S. meliloti is designated the cgm (cyclic glucan modification) locus, and the product encoded by ORF1 is referred to as CgmB.

Analysis of the Escherichia coli genome VI: DNA sequence of the region from 92.8 through 100 minutes

The 338.5 kb of the Escherichia coli genome described here together with previously described segments bring the total of contiguous finished sequence of this genome to > 1 Mb. Of 319 open reading frames (ORFs) found in this 338.5 kb segment, 147 (46%) are potential new genes. The positions of several genes which had been previously located here by mapping or partial sequencing have been confirmed. Several ORFs have functions suggested by similarities to other characterised genes but cannot be assigned with certainty. Fifteen of the ORFs of unknown function had been previously sequenced. Eight transfer RNAs are encoded in the region and there are two grey holes in which no features were found. The attachment site for phage P4 and three insertion sequences were located. The region was also analysed for chi sites, bend sites, REP elements and other repeats. A computer search identified potential promoters and tentative transcription units were assigned. The occurrence of the rare tetramer CTAG was analysed in 1.6 Mb of contiguous E.coli sequence. Hypotheses addressing the rarity and distribution of CTAG are discussed.