The dideoxy chain termination method using deoxy-7-deazaguanosine triphosphate (dc7GTP) in place of dGTP was found to be very useful. Sequencing of a part of the human N-myc gene having 85% GC content is impossible by the original method using dGTP, because of compression of bands. However, the nucleotide sequence of this part was unambiguously determined by analysis of both strands by the modified method. Use of dc7GTP is concluded to improve the dideoxy chain termination method for DNA sequencing.

Escherichia coli Ion mutants are defective in the ATP-dependent proteolysis of abnormal proteins. The mutants are also sensitive to ultraviolet light (UV) in that septation is inhibited after exposure to UV. sulA mutations, isolated as suppressors of UV sensitivity unlinked to ion, do not affect proteolysis but allow septation to occur after DNA damage. We have confirmed the hypothesis that the product ofthe sulA gene is degraded by ion proteolysis. If sulA (the product of sulA) is a UV-inducible division inhibitor, as suggested by a variety of experiments, lon (the product of ion) may regulate cell division by regulating the half-life of sulA. We cloned the sulA gene in a bacteriophage A vector from a plasmid carrying the ompA region ofE. coli. An 18-kilodalton polypeptide was identified as the product of the sulA gene. Pulse-chase labeling demonstrated that the half-life of the sulA protein is 1.2 min in ion' cells and 19 min in Ion-cells. This work demonstrates that lon proteolysis affects the stability of a native E. coli protein.The ion mutations of Escherichia coli affect a variety of physiological processes. Ion mutants have decreased ability to degrade abnormal proteins (1-4), overproduce capsular polysaccharide (5), and are defective for lysogeny of bacteriophages A and P1 (6, 7). They are also sensitive to DNA-damaging agents, such as ultraviolet light (UV) or methyl methanesulfonate (MeMes) (8). This UV sensitivity seems to result from an exaggeration of the normal inhibition of E. coli cell division after DNA damage (9). Ion mutants do not recover from this division arrest and are therefore UV and MeMes sensitive.Mutations identified in ion strains as suppressors of sensitivity to UV or MeMes map at two loci, sulA and sulB. These suppressors block the lethal filamentation seen in ion strains but do not affect other ion phenotypes (10-14). sulA and sulB also do not affect the cell's UV repair system or its induction. George and her co-workers (15) proposed that a division inhibitor, in-

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