Journal of General Microbiology ( I ~ T I) , 67, 107-1 15 Printed in Great Britain A Temperature-sensitive Cell Division Component in a Mutant of

Salmonella typhimurium strain 4 a is temperature-sensitive. It grows normally at 25 " but stops dividing immediately when shifted to 42". After the shift cell mass increases at a normal rate and DNA synthesis is unaffected. Viable count remains constant for about 3 h. and then falls off sharply. The filaments formed at 42' divide when returned to 25 " after a delay which in broth is about 70 min. This delay is not markedly dependent on the time previously spent at 42 " but is medium-dependent, being shorter in enriched media. Eventually all the divisions prevented at 42 ' take place at 25". Chloramphenicol almost completely stops filament septa-tion and division, and when added early in the recovery period causes fragility, some lysis and a sharp fall in viable count. Nalidixic acid does not stop filament division. It is concluded that a shift to 42 ' influences cell division directly by causing the irreversible inactivation of a component required at a late stage of division. The length of the division recovery period when filaments are returned to 25" suggests that the component must accumulate throughout most of the division cycle before division can restart