The synthesis of protein(s) for chromosome condensation may be regulated by a post-transcriptional mechanism.

A temperature sensitive mutant of BHK21, tsBN2, showed a premature chromosome condensation (PCC) upon the temperature shift of 40.5 degrees, even in the absence of DNA replication. The induction of PCC requires new protein synthesis, but not necessarily new RNA synthesis. Our data suggested that the messenger RNA for chromosome condensation starts to be transcribed at the beginning of S phase. At the permissive temperature (33.5 degrees), the messenger RNA for chromosome condensation translated with a very slow rate during S phase and rapidly in G2-M phase. At the nonpermissive temperature (40.5 degrees), however, those messenger RNAs were translated anytime, so that various figures of PCC appeared depending on the cell cycle. On the way of PCC induction, ribosomal RNA synthesis was inhibited at first, as expected from mitosis. Our data suggested that the synthesis of protein(s) for chromosome condensation was regulated by the post-transcriptional mechanism, in which tsBN2 might be defective, especially at the translational level.