Variation in chiasma frequency in male Trichosurus vulpecula (Marsupialia: Mammalia)

Chiasma frequency in spermatocytes was found to show extensive variation between individual males from a natural population of the marsupial Trichosurus vulpecula. Two factors which might contribute to the observed variation were investigated; season and age. However, neither of these had any significant effect on chiasma frequency or chiasma frequency variation.     

Trans splicing of mRNA precursors in vitro

Two exon segments from two separate RNA molecules can be joined in a trans splicing process. In trans splicing reactions, an RNA molecule containing an exon, a 5' splice site, and adjacent intron sequences was mixed with an RNA molecule containing an exon, a 3' splice site, and adjacent intron sequences. The efficiency of trans splicing of these two RNAs increased if the two termini of the intervening sequences were paired in a short RNA duplex. However, trans splicing of two RNA molecules with no significant complementarity was also observed. These results strongly suggest that significant secondary structures within intervening sequences could affect the splicing of flanking exons. Similarly, RNAs that are complementary to segments within the intervening sequences could potentially regulate the selection of splice sites. Finally, some organisms might use trans splicing to distribute a single exon to many different mRNAs.     

Electrophoretic separation of complexes involved in the splicing of precursors to mRNAs

Splicing complexes were analyzed by electrophoresis on a native low-percentage polyacrylamide gel. Two distinct heparin-resistant complexes, A and B, are assembled specifically on an RNA precursor containing authentic 5' and 3' splice sites. This assembly is ATP-dependent. Kinetic experiments suggest that complex A is converted with time to a larger, slower migrating complex B. Complexes A and B detected by gel electrophoresis correspond to material sedimenting at 25S and 35S, respectively. Substrate RNA containing only the 3' splice site is capable of forming the smaller complex A but not complex B. Complex A protects sequences upstream of the 3' splice site, encompassing the branch site and polypyrimidine tract from digestion by RNAase T1. U2 snRNA, but not U1 snRNA was detected in both complexes A and B by Northern hybridization analysis. Interestingly, an endogenous large complex containing U2 snRNP could be detected in nuclear extracts.     

Growth maintenance of the maize primary root at low water potentials involves increases in cell-wall extension properties, expansin activity, and wall susceptibility to expansins

Previous work on the growth biophysics of maize (Zea mays L.) primary roots suggested that cell walls in the apical 5 mm of the elongation zone increased their yielding ability as an adaptive response to low turgor and water potential (psi w). To test this hypothesis more directly, we measured the acid-induced extension of isolated walls from roots grown at high (-0.03 MPa) or low (-1.6 MPa) psi w using an extensometer. Acid-induced extension was greatly increased in the apical 5 mm and was largely eliminated in the 5- to 10-mm region of roots grown at low psi w. This pattern is consistent with the maintenance of elongation toward the apex and the shortening of the elongation zone in these roots. Wall proteins extracted from the elongation zone possessed expansin activity, which increased substantially in roots grown at low psi w. Western blots likewise indicated higher expansin abundance in the roots at low psi w. Additionally, the susceptibility of walls to expansin action was higher in the apical 5 mm of roots at low psi w than in roots at high psi w. The basal region of the elongation zone (5-10 mm) did not extend in response to expansins, indicating that loss of susceptibility to expansins was associated with growth cessation in this region. Our results indicate that both the increase in expansin activity and the increase in cell-wall susceptibility to expansins play a role in enhancing cell-wall yielding and, therefore, in maintaining elongation in the apical region of maize primary roots at low psi w.