Worm Breeder's Gazette 12(5): 38 (February 1, 1993)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

It's All in the timing; Imitiation and Maintenance of Homeotic Gene Expression

Craig Hunter, Cynthia Kenyon

UCSF, San Francisco, CA 94143-0554

The homeobox gene pal-1 functions in the V6 seam cell to prevent signals from neighboring cells from inhibiting mab-5 function in V6 (Waring and Kenyon 1990;1991). In both mab-5 and pal-1 mutants V6 generates a lineage similar to V1 -V4,producing alae instead of rays. V6 will make rays in pal-1 mutants if seam cells adjacent to V6 are ablated or if mab-5 activity is provided by the mab-5 gain-of-function allele e1751 .Steve Salser (see this WBG) has shown by immunofluorescence antibody staining that, unlike in WT, in pal-1 ( e2091 ), mab-5 protein is not expressed in V6 at hatching; a mab-5 -lacZreporter gene is also not expressed in V6 in the embryo. The current model is that signals between seam cells inhibit mab-5 expression in the seam cells and that pal-1 functions in V6 to overcome or bypass this inhibition.

We have asked when cell signals, mab-5 ,and pal-1 function. To learn when cell signals function, we ablated the posterior seam cell T, or its seam cell descendants T.a or T.ap, at one hour intervals following hatching in a pal-1 background. V6 descendants produced rays following ablation of T, T.a, and T.ap until about eight hours after hatching. Ablating T.ap after eight hours failed to rescue the pal-1 phenotype. Thus, in a pal-1 mutant, signals from T.ap inhibit mab-5 function in V6 and the effect of these signals becomes "fixed" at about eight hours post hatching.

To learn when mab-5 functions in V6 ,we expressed a mab-5 cDNA under heat shock control (Salser and Kenyon 1992). Heat shock-induced mab-5 expression before eight hours post hatching did not significantly rescue pal-1 whereas mab-5 expression after eight hours rescued most animals. That is, mab-5 can only function after eight hours.

We know that mab-5 is normally expressed in V6 beginning during embryogenesis (Cowing and Kenyon 1992), and that pal-1 activates mab-5 at this time since a mab-5 -lacZreporter construct is not expressed in V6 in pal-1 ( e2091 )embryos. To learn when pal-1 can function, we expressed a pal-1 cDNA under the control of the hsp16 -1heat shock promoter. Heat shocked-induced pal-1 expression anytime during embryogenesis or up to about eight hours after hatching rescued pal-1 mutants. Heat shock treatment after eight hours failed to rescue pal-1 .Since the function of pal-1 is to turn on mab-5 before the effect of the cell signals is fixed, and yet mab-5 functions after that pint, we conclude that the mab-5 gene must be expressed or made expressible before the effect of the cell signals becomes fixed. This observation suggests that homeotic genes not activated early are repressed and raises the possibility that there is a system for maintaining the expression states of homeotic genes in C. elegans, perhaps similar to the polycomb class of genes in Drosophila (see Maloof and Kenyon this WBG).

pal-1 may act by either inhibiting the cell signals (by repressing genes required for signal transduction) or by inhibiting their effect (perhaps mediating or preventing a change in the chromatin structure of mab-5 ).To identity other genes involved in this process we have begun isolating extragenic suppressor mutations of pal-1 in a visual screen of F2 clones. In a pilot experiment we isolated seven suppressors from 1500 clones. The three best characterized suppressors cause seam cells anterior to V5 to produce rays and cause anterior expression of a mab-5 -lacZreporter construct.