The actions of enzymes on lampbrush chromosomes

Abstract

ABSTRACT The chromomeres of lampbrush chromosomes of Triturus cristatus are Feulgen-positive; they therefore contain DNA. After removal of their DNA in boiling trichloracetic acid, the chromomeres stain with fast green at alkaline pH; they therefore contain basic protein. The lateral loops are Feulgen-negative; they stain with toluidine blue at acid pH, but much less intensely following RNase digestion; they therefore contain RNA. The spheres of chromosomes V and VIII do not contain RNA. Unfixed lampbrush chromosomes retain a life-like appearance in 0·07 M K/NaCl at pH 6·2; in this medium the nuclear sap disperses. As pH is raised to 8·5 the matrices of lateral loops dissolve but chromosome axes remain unbroken. Above pH 8·5 lampbrush chromosomes dissolve. As pH is lowered from 6·2, at between 5·8 and 5·4 coagulation occurs. If pH is rapidly reduced still further, a persistent relaxed condition sets in between 2·5 and 2. In concentrations of K/NaCl above 0·5 M lampbrush chromosomes dissolve. Lateral loop matrices dissolve in 0·25 M K/NaCl but chromosome axes remain unbroken. In concentrations of K/NaCl below 0·05 M lateral loop matrices dissolve, but even in distilled water chromosome axes remain unbroken. Trypsin at pH 6·2 and at pH 7·8 strips the matrices from lateral loops and occasionally breaks matrix fusions. It causes chromomeres to swell and coalesce, but fails to break chromosome axes. The action of ‘pan-protease’ resembles that of trypsin in all respects. Pepsin at pH 6·2 strips the matrices from lateral loops, but does not destroy chromomeres. At low pH peptic digestion is slow: the enzyme is attacking coagulated chromosomes; but if peptic digestion precedes a lowering of pH the overall outcome is a rapid solution of loop matrix, and under these conditions matrix and sphere fusions are broken. If trypsin or ‘pan-protease’ digestion precedes a lowering of pH there is a similarly rapid solution of loop matrix; thus the action is not specifically referable to pepsin. Under no conditions does pepsin break the axes of lampbrush chromosomes. RNase at pH 6·2 strips the matrices from lateral loops; this action is detectable at extreme dilution. RNase does not destroy chromomeres, nor does it break chromosome axes. If tryptic digestion follows RNase digestion this too fails to break chromosome axes. Unlike the proteolytic enzymes and RNase, DNase at pH 6·2 breaks the fibril between adjacent chromomeres, and it also breaks the axes of lateral loops. Contrary to Mazia’s experience with salivary gland chromosomes, versene does not break the axes of lampbrush chromosomes even when applied in media of low electrolyte concentration. These results indicate that uninterrupted fibres of DNA run throughout the lengths of lampbrush chromosomes. Contents