ROBERTSONIAN METACENTRICS OF THE HOUSE MOUSE LOSE TELOMERIC SEQUENCESBUT RETAIN SOME MINOR SATELLITE DNA IN THE PERICENTROMERIC AREA

A combination of cytogenetic and molecular biology techniques were use d to study the molecular composition and organisation of the pericentr omeric regions of house mouse metacentric chromosomes, the products of Robertsonian (Rb) translocations between telocentrics. Regardless of whether mitotic or meiotic preparations were used, in situ hybridisati on failed to reveal pericentromeric telomeric sequences on any of the Rb chromosomes, while all metacentrics retained detectable, although r educed (average 50 kb), amounts of minor satellite DNA in the vicinity of their centromeres. These results were supported by slot blot hybri disation which indicated that mice with 2n=22 Rb chromosomes have 65% of telomeric sequences (which are allocated to the distal telomeres of both Rb and telocentric chromosomes and to the proximal telomeres of telocentrics) and 15% the amount of minor satellite, compared with mic e with 2n=40 all-telocentric chromosomes. Pulsed field gel electrophor esis and Southern analysis of DNA from Rb mice showed that the size of the telomeric arrays is similar to that of mice with all-telocentric chromosomes and that the minor satellite sequences were hybridising to larger fragments incorporating major satellite DNA. Since the telomer ic sequences are closer to the physical end of the chromosome than the minor satellite sequences, the absence of telomeric sequences and the reduced amount of minor satellite sequences at the pericentromeric re gion of the Rb metacentrics suggest that the breakpoints for the Rb tr anslocation occur very close to the minor satellite-major satellite bo rder. Moreover, it is likely that the minor satellite is required for centromeric function, 50-67 kb being enough DNA to organise one centro mere with a functionally active kinetochore.