A DISTINCT CYCLIN-A IS EXPRESSED IN GERM-CELLS IN THE MOUSE

In this paper, the existence of two A-type cyclins in the mouse is dem onstrated. In the adult mouse, the expression of cyclin A1, which has greatest sequence identity with Xenopus cyclin A1, is restricted to ge rm cells, In contrast cyclin A2, which has greatest sequence identity with human cyclin A and Xenopus cyclin A2, is expressed in all tissues analysed. In order to explore the function of cyclin A1 in germ cells , its expression during the meiotic cell cycle and its associated kina se subunits have been characterised in the testis. The levels of cycli n A1 mRNA rise dramatically in late pachytene spermatocytes and become undetectable soon after completion of the meiotic divisions; thus its expression is cell cycle regulated. In lysates of germ cells from adu lt testes, cyclin A1 is present in p13(suc1) precipitates, and cyclin A1 immunoprecipitates possess histone H1 kinase activity, Three kinase partners of cyclin A1 were identified: p34(cdc2), a polypeptide of 39 x10(3) M(r) that is related to p33(cdk2) and, in lesser quantities, p3 3(cdk2). Cyclin A1 was also detected in oocytes; in metaphase I and me taphase II oocytes, a proportion of the cyclin A1 co-localises with th e spindle, possibly suggestive of a functional interaction. These data indicate that mammalian germ cells contain cyclin A1-dependent kinase s that either act as a substitute for, or in addition to, the cyclin A 2-dependent kinases characterised in somatic tissues.