Reg. MicrobioL 1991. 142, 195-200

(~ INgTITOTPASTEuR/ELSEVIER Paris 1991

Characterization of Escherichia coli mutants with altered ploidy N . J . T r u n ~'~ a n d S. G o t t e s m a n

Laboratory o f Molecular Biology, National Cancer Institute, National Institutes o f Health. Bethesda, MD 20892 (USA)

SUMMARY We describe the isolation and characterization of new mutants in the cell cycle of Escherlchia coil The mutants were selected as gain of function mutants that are able to maintain more than the normal number of chromosomes. Our increased ploldy mutants were isolated as resistant to camphor vapours, which selects for cells with more chromosomes than normal. The mutants (called mbr for molh-beg. resistant) map to four chromosomal locations: mbrA at 68 rain; mbrB at 88.5 rain; mbrC at 89.5 rain; and mbrD at 90 rain. To investigate the nature ~ these cell cycle mutants, we have coupled them with defects in recA, to test for induction of the SOS response, and dam, to determine if methylation is required for mbr function. Based on the results ol these and other tests, we have made a preliminary placement of the mbr mutants within the contsxt of the cell cycle, mbrA mutations appear to be defective in the coupling of the I)NA replication cycle to the cell (~lvision cycle, and as such, may define a new link between the two processes, mb,'E does not seem to be able to coordinate the cell cycle and the growth rate of the cell. mbrC appears to be defective in partitioning of chromosomes, mbrD, which may be allelic to rpoB (a subunit of RNA polyrnerase), appears to be defective in either chromosomal partitioning or the later stages of DNA replication. Key-words : E. coil, Camphor, Resistance, Ploidy, Mutant; recA, dam, Characterization. INTRODUCTION In Escherichia colt. the cell division cycle is characterized by a precisely timed sequence of events. A newly born cell must firs~ reach a critical mass, termed the initiation mass (Donaehie, 1968), before it begins D N A replication from a precise point on the chromosome, uriC (for review, ~.ee yon Meyenburg and Hanson, 1987). Replication then proceeds bidirectionally around

the chromosome until it reaches terC, where termination of DNA replication takes place (Kuempet et aL, 1977, Louarn et al., 1977). Elongation of the cell occurs continuously throughout D N A replication (Donachie and Begg, 1989). Once the DNA has been duplicated, the c h r o m o s o m e s must be physically moved to the two ends of the mother celt so that the septum can be formed between the two chromosomes. Following septum formation, the

(*) Correspondingauthor : NCLNIH, Bldg. 37, Rm. 4B03, Bethesda. MD 20[z92.

daughter cells are separzted into two distinct cells and the cycle begins anew. Recently, it has been proposed that the cell division and DNA replication cycles ran in parallel but independently o f one another (Bernander and Nordstrom, 1990). Because o f the fidelity of the cell division cycle, this implies that the two cycles must c o m m u n i c a t e with one another to insure production of viable daughter cells One level

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Characterization of Escherichia coli mutants with altered ploidy.

We describe the isolation and characterization of new mutants in the cell cycle of Escherichia coli. The mutants were selected as gain of function mut...
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