Expression of cyclin A can induce S phase. Embryos were pulse-labeled with BrdU for 30 min to visualize DNA synthesis. Embryos were collected for 1 h, aged for 7.5 h and heat-shocked (HS) for the times indicated. After a recovery period of 20 min, embryos were labeled with BrdU for 30 min, then fixed. Incorporated BrdU was detected with indirect immunofluorescence and analyzed on a confocal microscope. Optical section are shown from (a,c,e) the epidermal cell layer and from (b,d,f) the internal cell layers of the developing organs. (a,b) Wild-type control embryos show normal BrdU incorporation in the central nervous system including the brain (Br), the anterior and posterior midgut (AMG and PMG, marked with arrows), the hindgut and malphigian tubules. In the central midgut (CMG, marked with a star) some cells finish S phase 17. No S phase can be seen in the epidermis. (c,d) Embryos carrying two copies of a HS-DmcycA transgene were heat-shocked for 15 min. Only a few cells in the epidermis were induced to go into S phase. In the internal tissues, the normal spatial and temporal program of S phases is disturbed and in all cells of the midgut and the salivary gland (marked with a triangle) S-phase induction can be seen. (e,f) A heat-shock of 30 min induces S-phase in cells of the epidermis. Cells of the ventral epidermis show a slightly stronger label than cells of the dorsal epidermis. In the internal tissues, a homogeneous S phase is induced. (g,h) Schematic drawings of the embryos in (e,f) Abbreviations: Br, brain; DE, dorsal epidermis; VE, ventral epidermis; SG: salivary gland; AMG, anterior midgut; CMG, central midgut; PMG, posterior midgut.

Cyclin A is competent to drive S phase in the absence of cyclin E. BrdU pulse labelling of control embryos that received no heat-shock (a,c,e) and embryos of the same genotypes after heat-shock treatment (b,d,f). BrdU pulse labelling was performed at the indicated times after heat-shock treatment and on embryos of equivalent age that received no heat shock treatment. (a,b) Embryos carrying two copies of HS-DmcycA. S-phase induction by cyclin A can be observed in DmcycE⁺ embryos in internal tissues (not shown) and the epidermis 1 h after heat-shock. (c–f) Embryos homozygous for DmcycE^AR95 carrying two copies of HS-DmcycA. In embryos mutant for DmcycE, S-phase induction by cyclin A can be observed after 1 h in the endoreduplicating tissues (d), but epidermal S-phase induction is not seen until 180 min after S-phase induction (f).

Cyclin A is unstable in G1, but it can induce its own stabilization. Immunostaining for cyclin A is shown at internal and superficial focal planes for embryos 20 min or 150 min after heat-shock. (a–d) The pattern of cyclin A expression in heat-shocked control embryos. As in undisturbed embryos of this stage, few cells express cyclin A: the mitotically active neuronal cells (arrows) are not arrested in G1 and are actively expressing cyclin A; the quiescent germ cells are arrested in G2 and retain maternally expressed cyclin A (T.T. Su, and P.H.O'F., unpublished observations). (e–h) Cyclin A is unstable in G1 cells. The cyclin A expressed from two copies of a HS-DmcycA transgene in response to a short heat-shock (15 min) accumulates throughout the embryo: staining is seen in the epidermis (e), and internal tissues such as the salivary gland (triangle) and the hindgut (arrow) (f). The cyclin A signal is lost in many cells of the embryo by 150 min following the heat-shock (compare (g) with (e)). Cells that exit G1 and enter S phase retain cyclin A at the 150 min time – for example, the internal tissues (h). The few cells of the epidermis that continue to stain for cyclin A at 150 min probably represent exceptional cells that were induced into S phase by the low level of cyclin A induction (see ). This result suggests that cyclin A is considerably more stable in cells that have progressed to S phase than it is in G1 cells. (i–l) Cyclin A levels that induce G1 cells into S phase also induce stabilization of cyclin A. A 30 min heat-shock of embryos carrying two copies of a HS-DmcycA transgene induces high levels of cyclin A (i,j), and widespread S phase (). In contrast to the instability of cyclin A in the epidermis following a 15 min heat-shock (g), the cyclin A that was induced in response to this longer heat-shock persisted in most of the cells of the epidermis (k), as well as persisting in the internal tissues (l). There are a few cells that so not stain for cyclin A after 150 min (see black holes in the staining in panel (k)): these may represent cells that were not induced into S phase by the treatment. The difference between the middle and the bottom series of panels suggests that cyclin A can stabilize itself, and the conditions that lead to stabilization are the same as those that lead to induction of G1 cells into S phase (see ).

S-phase induction by cyclin A^s alone and in combination with cdc2^AF. (a–r) Embryos were pulse-labeled with BrdU and incorporation visualized with indirect immunofluorescence. Optical sections of the epidermal cell layers are shown to detect S-phase induction in the epidermis. (a–f) A heat shock of more than 10 min is needed to induce S phase in the epidermis of embryos carrying two copies of the cyclin A transgene. (b,d,f) Higher magnifications of the embryos shown in (a,c,e), respectively. (g–l) In embryos carrying one copy of the cyclin A^s heat-shock transgene, S phase can be induced after a 10 min heat-shock. (m–r) Coexpression of cyclin A^s and cdc2^AF can induce S phase after a 5 min heat-shock. (s,t) Expression of cyclin A and cyclin A^s from the heat-shock transgene after varying the heat-shock length. Cyclin A proteins are detected on immunoblots using a monoclonal antibody. Each lane contains the protein from one embryo, each carrying two copies of the respective transgene. Embryos were heat-shocked for the indicated times and fixed with methanol approximately 5 min after the heat-shock. The two blots were developed separately and the blot in (t) had to be exposed about 5-times longer to see the cyclin A^s signal. (u) Schematic presentation of the cyclin constructs. Cyclin A is the full-length Drosophila cyclin A. The cyclin box is striped and the destruction box is filled. Cyclin A^s lacks the first 169 amino acids, including the destruction box.

Expression of cyclin A results in inhibitory phosphorylation of cdc2. Total protein extracts from single embryos containing two copies of the HS-DmcycA transgene and one copy of the HS-Dmcdc2 gene were separated on SDS-PAGE and immunoblotted using a polyclonal anti-cyclin A and anti-cdc2 antibody respectively. Embryos were either not heat-shocked (0 time points) or were collected after a 30 min heat-shock at the times indicated. For each time point, two embryos were analyzed. Phosphorylation of cdc2 requires the binding of cyclin and neither cyclin A or cyclin B are normally expressed in the majority of the cells at this stage in development (0 min time points). Expression of cyclin A from the heat-shock promoter results in the appearance of slower migrating forms of cdc2 which are indicative of Y15 and/or T14 phosphorylated forms of cdc2.

In rux mutant embryos, cyclin A and cyclin A^s are more effective in inducing S phase, but overexpression of rux can inhibit S phase function of cyclin A^s completely. (a–n) Embryos were pulse-labeled with BrdU and incorporation visualized with indirect immunofluorescence. Optical section were taken on a confocal microscope to determine S-phase induction in the epidermis. (a–f) In rux mutant embryos, induction of cyclin A from one copy of HS-DmcycA is sufficient to induce S-phase after a 10 min heat-shock (compare to ). (g–l) In rux mutant embryos, S-phase induction by cyclin A^s is enhanced, causing S-phase induction after a 5 min heat-shock. (m,n) Overexpression of rux can block the ectopic S-phase induction of cyclin A^s completely. Normal S phases of the central nervous system are apparently not affected by rux overexpression. (o) Immunoblots of single embryos extracts collected 30 and 60 min after a 30 min heat-shock and developed for cyclin A (A) and cyclin A^s (B), respectively. Lane 1: control embryos that received no heat-shock show little endogenous cyclin A. Lanes 2,3: cyclin A and cyclin A^s accumulate after heat shock and levels appear stable 30 min and 60 min after the heat shock. Lane 4: coexpression of cyclin A or cyclin A^s with rux; 30 min after heat-shock, cyclin A^s accumulates to similar levels as seen in lane 2 and cyclin A is present already at lower levels. Lane 5: 60 min after coexpression with rux, cyclin A levels have diminished but cyclin A^s levels have increased.

Rux causes cyclin A to be degraded and cyclin A^s to accumulate in the nucleus. Optical sections of whole embryos stained with a polyclonal antibody to cyclin A (αA) or stained with propidium iodide to visualize DNA. Embryos were 7.5–8.5 h old and heat-shocked for 30 min. After 60 min, embryos were fixed. Cyclin A proteins were detected using a polyclonal cyclin A antibody and a secondary FITC-labeled antibody. Optical sections are shown that focus on epidermal cells of thoracic and abdominal segments. (a) Embryo carrying two copies of HS-DmcycA Most cells shown are in G1 and accumulate cyclin A mainly in the cytoplasm. The inset shows a region of the first thoracic segment with some cells accumulating higher levels of cyclin A. These cells also have bigger nuclei and are believed to be in G2. Immediately above those cells are a few cells with no cyclin A staining. Presumably, those cells have just completed mitosis. It can be seen that the cells at the top of the inset also have some, although weak, nuclear staining of cyclin A. (b) Embryos with one copy of HS-DmcycA^s. The expression of cyclin A^s appears almost uniform throughout the cells, including the nucleus. (c) This embryo has two copies of the HS-DmcycA and one copy of the HS-rux transgene. In almost all cells, cyclin A is expressed at very low levels with some residual expression seen in the cytoplasm. The stronger staining cells are likely neuroblasts in which cyclin A localization and accumulation is apparently not affected by the expression of rux. (d) In embryos that carry one copy of HS-DmcycA^s and one copy of HS-rux, cyclin A^s shows distinct nuclear accumulation.