Two functionally dependent acetylcholine subunits are encoded in a single Caenorhabditis elegans operon.

The deg-3 gene from the nematode Caenorhabditis elegans encodes an alpha subunit of a nicotinic acetylcholine receptor that was first identified by a dominant allele, u662, which produced neuronal degeneration. Because deg-3 cDNAs contain the SL2 trans-spliced leader, we suggested that deg-3 was transcribed as part of a C. elegans operon. Here we show that des-2, a gene in which mutations suppress deg-3(u662), is the upstream gene in that operon. The des-2 gene also encodes an alpha subunit of a nicotinic acetylcholine receptor. As expected for genes whose mRNAs are formed from a single transcript, both genes have similar expression patterns. This coexpression is functionally important because (i) des-2 is needed for the deg-3(u662) degenerations in vivo; (ii) an acetylcholine-gated channel is formed in Xenopus oocytes when both subunits are expressed but not when either is expressed alone; and (iii) channel activity, albeit apparently altered from that of the wild-type channel, results from the expression of a u662-type mutant subunit but, again, only when the wild-type DES-2 subunit is present. Thus, the operon structure appears to regulate the coordinate expression of two channel subunits.

Atrioventricular canal malformation interpreted as secondary to reduced compression upon the developing heart.

This study was undertaken to evaluate the nature and pathogenesis of malformations of the atrioventricular canal in relation to normal cardiogenesis. Serial histologic sections of normal human embryos and fetuses were made, from which three-dimensional images were reconstructed to show the relationship between the developing heart and its surrounding structures, and the course of development of the atrial septum and atrioventricular valves. Based on these reconstructions and on examination of the hearts of 59 patients with atrioventricular canal malformations, it is suggested that the spectrum of atrioventricular malformations may arise as a result of reduced compression of the developing atria by surrounding structures during embryonic Stages 13 through 18. Comparison of hearts with atrioventricular canal defects with normal embryos indicated that the malformations may be classified as primitive canals, complete canals, or partial canals, corresponding to failure of completion of normal development in Stages 14 through 18. In primitive canal the atrial septum was absent or had only a portion of septum primum. In complete canal both atrial septums were present, but the atrioventricular valve material was not subdivided and the four chambers were in communication. In partial canal, the atrioventricular valve was divided, but atrial and ventricular septal defects and valve clefts were present in varying degrees of severity. It is proposed that the spectrum of cardiac abnormalities which constitutes atrioventricular canal malformations may be understood as arising from varying degrees of lack of normal compression of the developing heart by surrounding structures. (Am J Pathol 95.579-598, 1979)