INTRODUCTION Progression through the cell cycle is controlled by the sequen- tial activation of a set of cyclin-dependent kinases (cdks). The minimal components found in an active cdk complex are a catalytic subunit related to the product of the Schizosaccha- romyces pombe gene cdc2 and a cyclin. During the cell cycle, periodic fluctuations in the concentrations of cyclins underlie the sequential activation of most cdks. Association with a cyclin is necessary, but not always sufficient, for activation of the kinase, which is also regulated by phosphorylation of tyrosine and threonine residues and by inhibitor polypeptides. It is believed that the activity of a particular complex is necessary to pass through a boundary separating cell cycle events (reviewed in Pines, 1993). This is clearly the case for the G 2 to M transition which is regulated by the activation of cyclin B-p34 cdc2 . As different cdks catalyse different tran- sitions, it is assumed that individual cyclin-cdk complexes have different substrate specificities. One clearly demonstrated example of this is the phosphorylation of the retinoblastoma- related protein p107 by cyclin A-cdk but not by cyclin B-cdk (Peeper et al., 1993). Cyclin A, along with cyclin B, was first identified in the oocytes and cleaving embryos of marine invertebrates as polypeptides that accumulate during interphase but are abruptly destroyed at each mitotic (Evans et al., 1983; Swenson et al., 1986; Luca and Ruderman, 1989; Hunt et al., 1992) or meiotic division (Swenson et al., 1986; Standart et al., 1987). Cyclin B-p34 cdc2 is maturation promoting factor (MPF), responsible for the resumption of meiosis during oocyte mat- uration (Dunphy and Newport, 1988; Gautier et al., 1988, 1990; Draetta et al., 1989; Labbe et al., 1989). While MPF activity is necessary for entry into mitosis, its inactivation is a prerequisite for exit (Murray and Kirschner, 1989). In contrast to the role of cyclin B in the control of the G 2 to M transition, the function of the cyclin A-associated kinase(s) remains obscure. In Xenopus early embryos, cyclin A1 associ- ates predominantly with p34 cdc2 , although a small fraction binds the related kinase subunit p33 cdk2 (Kobayashi et al., 1992). In cleaving embryos of the surf clam, destruction of cyclin A precedes that of cyclin B which occurs at the metaphase-anaphase transition (Luca and Ruderman, 1989; Hunt et al., 1992). Similarly, in Xenopus, cyclin A1-associated kinase is inactivated a few minutes before the cyclin B1- or B2-associated kinase (Minshull et al., 1990). The A- and B- type cyclins can be further distinguished by the conditions 53 Development 122, 53-64 (1996) Printed in Great Britain © The Company of Biologists Limited 1996 DEV3209 In this paper, the existence of two A-type cyclins in the mouse is demonstrated. In the adult mouse, the expression of cyclin A1, which has greatest sequence identity with Xenopus cyclin A1, is restricted to germ 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 kinase subunits have been characterised in the testis. The levels of cyclin 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 adult testes, cyclin A1 is present in p13 suc1 precipi- tates, and cyclin A1 immunoprecipitates possess histone H1 kinase activity. Three kinase partners of cyclin A1 were identified: p34 cdc2 , a polypeptide of 39×10 3 M r that is related to p33 cdk2 and, in lesser quantities, p33 cdk2 . Cyclin A1 was also detected in oocytes; in metaphase I and metaphase II oocytes, a proportion of the cyclin A1 co- localises with the spindle, possibly suggestive of a func- tional interaction. These data indicate that mammalian germ cells contain cyclin A1-dependent kinases that either act as a substitute for, or in addition to, the cyclin A2- dependent kinases characterised in somatic tissues. Key words: mouse, cyclin A, germline, spermatogenesis, oocyte SUMMARY A distinct cyclin A is expressed in germ cells in the mouse Claire Sweeney 1 , Martin Murphy 1,‡ , Michal Kubelka 2,† , Stuart E. Ravnik 3 , Chris F. Hawkins 1 , Debra J. Wolgemuth 3 and Mark Carrington 1, * 1 University of Cambridge, Department of Biochemistry, Cambridge, CB2 1QW, UK 2 The Babraham Institute, Cambridge, CB2 4AT, UK 3 Department of Genetics and Development, Columbia University, New York, NY 10032, USA *Author for correspondence † Permanent address: Institute of Animal Physiology and Genetics, Department of Genetics, Academy of Sciences of the Czech Republic, 277 21 Libechov, Czech Republic ‡ Present address: Faculté Necker, INSERM U370, 156 Rue de Vaugirard, Paris 75015, France