The hsd genes of E. coli K 12 have been cloned in phage lambda by a combination of in vitro and in vivo techniques. Three genes, whose products are required for K-specific restriction and modification, have been identified by complementation tests as hsdR, M and S. The order of these closely linked genes was established as R, M, S by analysis of the DNA of genetically characterised deletion derivatives of lambda hsd phages. The three genes are transcribed in the same direction but not necessarily as a single operon. Genetic evidence identifies two promoters, one from which transcription of hsdM and S is initiated and a second for the hsdR gene. The hsdR gene codes for a polypeptide of molecular weight approximately 130 000; hsdM for one of 62--65 000 and the hsdS gene was associated with two polypeptides of approximately 50 000. Circumstantial evidence suggest that one of these two polypeptides may be a degradation, or processed, derivative of the other. The hsdS polypeptide of E. coli B has a slightly higher mobility in an SDS-polyacrylamide gel than does that of E. coli K 12. A probe comprising most of the hsdR gene and all of the hsdM and S genes of E. coli K 12 shares extensive homology with the DNA of E. coli B but none with that of E. coli C.

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Histidine residues of actin were carbethoxylated with diethylpyrocarbonate. Four and three histidine residues per actin monomer were rapidly carbethoxylated in the G and F form, respectively. After partial carbethoxylation of G-actin a non-polymerizable fraction could be separated by ultracentrifugation. The analysis of these fractions showed that one of the four fast-reacting histidines of G-actin plays an essential role in polymerization.Removal of the carbethoxy groups from the histidine residues by hydroxylamine treatment restored the ability to polymerize.The specificity of the reaction was tested by the use of [14C]diethylpyrocarbonate. After tryptic digestion of non-polymerizable carbethoxylated actin a single histidyl peptide was isolated by gel filtration and diagonal electrophoresis. The histidine residue essential for polymerization was identified as histidine-40.The essential role of histidine residues in the polymerization of actin has been observed by several investigators in photooxidation experiments [I -41.We have shown earlier [5] that the ability of G-actin to polymerize considerably decreases on carbethoxylation of histidine residues, whereas carbethoxylated F-actin is not depolymerized spontaneously, and carbethoxylated G-actin obtained by depolymerization of diethylpyrocarbonate-treated F-actin can be polymerized. The number of histidine residues involved in the polymerization process was not determined in these experiments.