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The expression of genes adjacent to ptsI was investigated using a series of specialised transducing phages carrying different, overlapping, segments of the cysA‐gsr‐ptsI‐ptsH‐ iex ‐ cysZ ‐lig region of the genome of Escherichia coli. The polypeptides were synthesised following the infection of u.v.‐irradiated lysogenic and non‐lysogenic uvrA recA hosts or a uvrA recA host carrying the lambda cI+ plasmid pKB280 . The polypeptides were identified by SDS‐polyacrylamide gel electrophoresis and fluorography. The gsr gene product had a mol. wt. of 23 000. The product of the iex gene was tentatively identified as a protein of mol. wt. of either 33 000 or 21 000. Hpr, the product of the gene ptsH, had a mol. wt. of 9000. The gsr gene appeared to be expressed at a higher level in a non‐immune host, which suggests that it was transcribed from lambda promoters. A new lambda host strain, suitable for the detection of small polypeptides (mol. wt. less than 30 000) is described.

5′-Deoxyadenosylcobalamin-dependent methylmalonyl-CoA mutase was purified to homogeneity from Propionibacterium shermanii by a simplified procedure. The native enzyme has an apparent Mr of 165,000, similar to the enzyme from other sources but larger than previously reported. It consists of two non-identical subunits, of Mr 79,000 and 67,000 respectively. The smaller subunit is apparently not a proteolytic fragment of the larger one. The final preparation usually contained some inactive mutase, bearing a tenaciously bound cobalamin species. This protein proved to be readily separable from apoenzyme by fast protein liquid chromatography on anion-exchange columns.

The expression of genes adjacent to ptsI was investigated using a series of specialised transducing phages carrying different, overlapping, segments of the cysA‐gsr‐ptsI‐ptsH‐ iex ‐ cysZ ‐lig region of the genome of Escherichia coli. The polypeptides were synthesised following the infection of u.v.‐irradiated lysogenic and non‐lysogenic uvrA recA hosts or a uvrA recA host carrying the lambda cI+ plasmid pKB280 . The polypeptides were identified by SDS‐polyacrylamide gel electrophoresis and fluorography. The gsr gene product had a mol. wt. of 23 000. The product of the iex gene was tentatively identified as a protein of mol. wt. of either 33 000 or 21 000. Hpr, the product of the gene ptsH, had a mol. wt. of 9000. The gsr gene appeared to be expressed at a higher level in a non‐immune host, which suggests that it was transcribed from lambda promoters. A new lambda host strain, suitable for the detection of small polypeptides (mol. wt. less than 30 000) is described.