4 DISTINCT AND UNUSUAL LINKER PROTEINS IN A MITOTICALLY DIVIDING NUCLEUS ARE DERIVED FROM A 71-KILODALTON POLYPROTEIN, LACK P34(CDC2) SITES, AND CONTAIN PROTEIN KINASE-A SITES

Tetrahymena thermophila micronuclei contain four linker-associated pro teins, alpha, beta, gamma, and delta. Synthetic oligonucleotides based on N-terminal protein sequences of beta and gamma were used to clone the micronuclear linker histone (MLH) gene. The MLH gene is single cop y and is transcribed into a 2.4-kb message encoding all four linker-as sociated proteins. The message is translated into a polypeptide (Mic L H) that is processed at the sequence down RTK to give proteins whose a mino acid sequences differ markedly from each other, from the sequence of macronuclear H1, and from sequences of typical H1s of other organi sms. This represents the first example of multiple chromatin proteins derived from a single polyprotein. The delta protein consists largely of two high-mobility-group (HMG) boxes. An evolutionary analysis of HM G boxes indicates that the delta HMG boxes are similar to the HMG boxe s of tsHMG, a protein that appears in elongating mouse spermatids when they condense and cease transcription, suggesting that delta could pl ay a similar role in the micronucleus. The micronucleus divides mitoti cally, while the macronucleus divides amitotically. Surprisingly, macr onuclear H1 but not Mic LH contains sequences resembling p34cdc2 kinas e phosphorylation sites, while each of the Mic LH-derived proteins con tains a typical protein kinase A phosphorylation site in its carboxy t erminus.