Micrococcal nuclease is shown to cleave DNA under conditions of partial digestion in a specific manner. Sequences of the type 5'CATA and 5'CTA are attacked preferentially, followed by exonucleolytic degradation at the newly generated DNA termini. GC-rich flanking sequences further increase the probability of initial attack. Unexpectedly, long stretches containing only A and T are spared by the nuclease. These results, which were obtained with spared by the nuclease. These results, which were obtained with mouse satellite DNA and two fragments from the plasmid pBR22, do not support the previous contention that it is the regions of high At-content which are initially cleaved by micrococcal nuclease. This specificity of micrococcal nuclease complicates its use in experiments intended to monitor the nucleoprotein structure of a DNA sequence in chromatin.

The nucleotide sequence of uncloned mouse satellite DNA has been determined by analyzing Sau96I restriction fragments that correspond to the repeat unit of the satellite DNA. An unambiguous sequence of 234 bp has been obtained. The sequence of the first 250 bases from dimeric satellite fragments present in Sau96I limit digests corresponds almost exactly to two tandemly arranged monomer sequences including a complete Sau96I site in the center. This is in agreement with the hypothesis that a low level of divergence which cannot be detected in sequence analyses of uncloned DNA is responsible for the appearance of dimeric fragments. Most of the sequence of the 5% fraction of Sau96 monomers that are susceptible to TaqI has also been determined and has been found to agree completely with the prototype sequence. The monomer sequence is internally repetitious being composed of eight diverged subrepeats. The divergence pattern has interesting implications for theories on the evolution of mouse satellite DNA.

A 5.3 kb EcoRI fragment (T3, abbreviations in ref.2) has been cloned from DNA of a kappa light chain producing mouse myeloma. The fragment hybridizes to the 5' flanking sequences of the J1 gene segment but not to C gene sequences of kappa light chain DNA. Restriction nuclease mapping and partial nucleotide sequencing showed that the fragment consists of sequences from the 5' side of the Jl and from the 3' side of a V gene segment, which apparently had been linked in a genomic rearrangement process. These rearranged flanking sequences are not the flanking sequences of the V and J gene segments which had been joined to form the two kappa light chain genes of the myeloma. Fragments with the hybridization properties of T3 have been found also in two other kappa and one lambda chain producing myelomas. The linking of flanking sequences in the myeloma genome is discussed with respect to the mechanism of recombination between V and J gene segments. INTRODUCTIONRearrangement of immunoglobulin light chain gene segments results in the joining of V (variable) and J (joining) gene segments (3,4). Four mechanisms have been proposed for the V-J recombination (5). 1) The copy-insertion model proposes that a V gene segment is duplicated and inserted upstream of a J gene segment.2) The excision-insertion model assumes that the V gene segment itself is excised and then integrated. 3) According to the inversion model a DNA segment is inverted which results in the joining of a V gene segment to a J gene segment in the correct orientation. 4) The deletion model proposes that the DNA segment separating V and J gene segments in germline DNA is deleted in the course of the V-J joining.In sequencing studies with cloned V and J gene fragments a characteristic heptanucleotide sequence was found adjacent to

Leader, variable (V) and joining (J) gene segments, and adjacent regions of two rearranged alleles of the same kappa-chain producing mouse myeloma, comprising approximately 3,200 base pairs, have been sequenced. Sequence comparisons are reported. V-J joining in one of the alleles leads to a reading frame with a stop codon within the J-gene segments. Allelic exclusion is apparently realized in this tumour through the formation of such a non-functional allele.

Digestion of mouse liver nuclei with DNase II leads to a novel cleavage pattern with a 100-nucleotide pair periodicity. From chromatin, this pattern or the standard 200-nucleotide pair repeat can be produced depending on the ionic conditions. The results are interpreted by assuming different conformational states of the nuclear material, including condensed and extended forms.