Determination of probability of plasmid loss per generation

Identification and genetic mapping of CHL genes controlling mitotic chromosome transmission in yeast

Eight independent chl (chromosome loss) mutants were isolated using yeast haploid strain disomic for chromosome III. In these mutants, chromosome III is lost during mitosis 50-fold more frequently than in the wild-type strains. chl mutants are also incapable of stable maintenance of circular and linear artificial chromosomes. Seven of the eight mutations are recessive, and one is semidominant. Complementation tests placed these mutants into six complementation groups (chl11 through chl16). Based on tetrad analysis, chl12, chl14 and chl15 correspond to mutations in single nuclear genes. Tetrad analysis of the other mutants was not possible due to poor spore viability. Complementation analysis was also carried out between collection of chl mutants and ctf mutants (chromosome transmission fidelity) (Spencer et al., 1990). The chl3, chl4, chl8, chl12 and chl15 mutants were unable to complement ctf3, ctf17, ctf12, ctf18 and ctf4, respectively. Three CHL genes were mapped by tetrad analysis. The CHL3 gene is placed on the right arm of chromosome XII, between the ILV5 (33.3 cM) and URA4 (21.8 cM) loci. The CHL10 gene is located on the left arm of chromosome VI, 12.5 cM from the centromere. The CHL15 gene is tightly linked to the KAR3 marker of the right arm of chromosome XVI (8.8 cM). The mapping data indicate that these three genes differ from other genes known to affect chromosome stability in mitosis. Therefore, the total number of the CHL genes identified (including those described by us earlier) is 13 (CHL1-CHL10, CHL12, CHL14 and CHL15).

CTF4 (CHL15) mutants exhibit defective DNA metabolism in the yeast Saccharomyces cerevisiae

We have analyzed the CTF4 (CHL15) gene, earlier identified in two screens for yeast mutants with increased rates of mitotic loss of chromosome III and artificial circular and linear chromosomes. Analysis of the segregation properties of circular minichromosomes and chromosome fragments indicated that sister chromatid loss (1:0 segregation) is the predominant mode of chromosome destabilization in ctf4 mutants, though nondisjunction events (2:0 segregation) also occur at an increased rate. Both inter- and intrachromosomal mitotic recombination levels are elevated in ctf4 mutants, whereas spontaneous mutation to canavanine resistance was not elevated. A genomic clone of CTF4 was isolated and used to map its physical and genetic positions on chromosome XVI. Nucleotide sequence analysis of CTF4 revealed a 2.8-kb open reading frame with a 105-kDa predicted protein sequence. The CTF4 DNA sequence is identical to that of POB1, characterized as a gene encoding a protein that associates in vitro with DNA polymerase alpha. At the N-terminal region of the protein sequence, zinc finger motifs which define potential DNA-binding domains were found. The C-terminal region of the predicted protein displayed similarity to sequences of regulatory proteins known as the helix-loop-helix proteins. Data on the effects of a frameshift mutation suggest that the helix-loop-helix domain is essential for CTF4 function. Analysis of sequences upstream of the CTF4 open reading frame revealed the presence of a hexamer element, ACGCGT, a sequence associated with many DNA metabolism genes in budding yeasts. Disruption of the coding sequence of CTF4 did not result in inviability, indicating that the CTF4 gene is nonessential for mitotic cell division. However, ctf4 mutants exhibit an accumulation of large budded cells with the nucleus in the neck. ctf4 rad52 double mutants grew very slowly and produced extremely high levels (50%) of inviable cell division products compared with either single mutant alone, which is consistent with a role for CTF4 in DNA metabolism.

CTF4 (CHL15) mutants exhibit defective DNA metabolism in the yeast Saccharomyces cerevisiae

We have analyzed the CTF4 (CHL15) gene, earlier identified in two screens for yeast mutants with increased rates of mitotic loss of chromosome III and artificial circular and linear chromosomes. Analysis of the segregation properties of circular minichromosomes and chromosome fragments indicated that sister chromatid loss (1:0 segregation) is the predominant mode of chromosome destabilization in ctf4 mutants, though nondisjunction events (2:0 segregation) also occur at an increased rate. Both inter- and intrachromosomal mitotic recombination levels are elevated in ctf4 mutants, whereas spontaneous mutation to canavanine resistance was not elevated. A genomic clone of CTF4 was isolated and used to map its physical and genetic positions on chromosome XVI. Nucleotide sequence analysis of CTF4 revealed a 2.8-kb open reading frame with a 105-kDa predicted protein sequence. The CTF4 DNA sequence is identical to that of POB1, characterized as a gene encoding a protein that associates in vitro with DNA polymerase alpha. At the N-terminal region of the protein sequence, zinc finger motifs which define potential DNA-binding domains were found. The C-terminal region of the predicted protein displayed similarity to sequences of regulatory proteins known as the helix-loop-helix proteins. Data on the effects of a frameshift mutation suggest that the helix-loop-helix domain is essential for CTF4 function. Analysis of sequences upstream of the CTF4 open reading frame revealed the presence of a hexamer element, ACGCGT, a sequence associated with many DNA metabolism genes in budding yeasts. Disruption of the coding sequence of CTF4 did not result in inviability, indicating that the CTF4 gene is nonessential for mitotic cell division. However, ctf4 mutants exhibit an accumulation of large budded cells with the nucleus in the neck. ctf4 rad52 double mutants grew very slowly and produced extremely high levels (50%) of inviable cell division products compared with either single mutant alone, which is consistent with a role for CTF4 in DNA metabolism.

A direct selection procedure for isolating yeast mutants with an impaired segregation of artificial minichromosomes

The nondisjunction of artificial yeast minichromosomes (2:0 segregation events) during mitosis is accompanied by the appearance of cells containing more than one copy of the minichromosome. A mathematical simulation of this process has demonstrated that under certain conditions, a nondisjunction of the minichromosomes may result in their accumulation in a considerable portion of the cell population. An increase in the copy number of artificial minichromosomes as a result of impaired segregation has been used to develop a new experimental procedure for directly selecting yeast mutants showing an impaired segregation of artificial minichromosomes during mitosis. Four new genes, AMC1, AMC2, AMC3, and AMC4, which control the segregation of artificial minichromosomes in mitosis, have been identified (AMC3 and AMC4 are mapped to chromosome IV and VII, respectively). Mutations in the genes AMC1-AMC4 also affect the mitotic transmission of natural chromosomes. We suggest that the genes AMC1, AMC2, AMC3, and AMC4 control the segregation of natural chromosomes in yeast.

Genetic control of chromosome stability in the yeast Saccharomyces cerevisiae

We have identified four new genetic loci: CHL2 (on chromosome XII), CHL3 (on chromosome XII); CHL4 (on chromosome IV), and CHL5 (on chromosome IX), controlling mitotic transmission of yeast chromosomes. The frequency of loss of chromosomes is 10-100-fold higher in chl5, chl2, chl3 and chl4 mutants than observed in wild-type strains. The mutants also show unstable maintenance of artificial circular minichromosomes with various chromosomal replicators (ARS) and one of the centromeric loci (CEN3, CEN4, CEN5 or CEN6). The instability of minichromosomes in the chl5, chl2, and chl4 mutants is due to the loss of minichromosomes in mitosis (1:0 segregation). In the chl3 mutant the instability of artificial minichromosomes is due to nondisjunction (2:0 segregation). The CHL3 gene therefore appears to affect the segregation of chromosomes during cell division.

The stability of chromosomes in yeast

CL mutants with high instability of chromosome III were UV-induced in haploid strain disomic for chromosome III. The obtained CL mutants can be divided into two groups: (1) CL2, CL3, CL7, CL11-CL13 with elevated level of spontaneous inter- and intragenic recombination and (2) CL4, CL8 in which instability of chromosome III is not accompanied by elevation of mitotic recombination frequency. CL4 and CL8 mutants also show unstable maintenance of artificial minichromosomes with different chromosomal replicators and centromeric loci. The instability of chromosome III and minichromosomes in CL4 and CL8 is determined by two nonallelic genes designated ch14 and ch18. The role of ch14 and ch18 genes in mitotic chromosome transmission is discussed.

A mutant of Saccharomyces cerevisiae with impaired maintenance of centromeric plasmids

A mutant with unstable maintenance of hybrid plasmids containing either one of the centromeric loci CEN3, CEN6, CEN11 and ars1 or the replicator of the 2 mu plasmid has been obtained. The frequency of loss of hybrid plasmids in the mutant was up to 3.10(-1) per one generation versus 10(-2) in the original strain. The unstable maintenance of minichromosomes in the mutant is controlled by a recessive nuclear gene, named SMC for stability of minichromosomes. Loss of some minichromosomes is connected with impairment of their segregation in cell division. In diploids homozygous for smc mitotic chromosomal segregation is not affected but sporulation is impaired. The question of adequacy of usage of minichromosomes for selection of mutants with impaired function of centromeric loci is discussed.