Molecular cloning of cDNA encoding N-acetylglucosaminyltransferase II from Arabidopsis thaliana.

N-acetylglucosaminyltransferase II (GnTII, EC 2.4.1.143) is a Golgi enzyme involved in the biosynthesis of glycoprotein-bound N-linked oligosaccharides, catalysing an essential step in the conversion of oligomannose-type to complex N-glycans. GnTII activity has been detected in both animals and plants. However, while cDNAs encoding the enzyme have already been cloned from several mammalian sources no GnTII homologue has been cloned from plants so far. Here we report the molecular cloning of an Arabidopsis thaliana GnTII cDNA with striking homology to its animal counterparts. The predicted domain structure of A. thaliana GnTII indicates a type II transmembrane protein topology as it has been established for the mammalian variants of the enzyme. Upon expression of A. thaliana GnTII cDNA in the baculovirus/insect cell system, a recombinant protein was produced that exhibited GnTII activity.

Fixed-ratio and variable-ratio schedules of brief stimuli in second-order schedules of matching to sample.

Eight pigeons matched to sample under second-order schedules of food reinforcement. Under fixed-interval unit schedules, the first correct match to occur after a given period of time was followed by the presentation of a brief stimulus. The termination of the last fixed-interval unit schedule was followed by food according to second-order fixed-ratio and variable-ratio schedules. In Experiment 1, as the number of fixed-interval unit schedules increased, long pauses occurred under the second-order fixed-ratio schedules, but not under the variable-ratio schedules. The similarity of performance measures such as local rate and accuracy indicated that the differences engendered by these two types of schedule are in the duration of the periods of not-responding. In Experiment 2, the addition of a brief stimulus at the end of each unit schedule in chained schedules that had different discriminative stimuli present for the duration of each unit did not substantially affect the performance, and long pauses continued to occur. However, few long pauses occurred under schedules with brief stimulus presentations alone. The most inaccurate performances were engendered by chained schedules without brief stimuli.

Accuracy of performance on a matching-to-sample procedure under interval schedules.

Correct matches on a matching-to-sample procedure were reinforced under fixed-interval, chained fixed-interval, and fixed-interval schedules with exteroceptive stimulus changes correlated with time since the last reinforcer (an added clock). For all four pigeons, accuracy changed within the fixed-interval and fixed-interval schedules with added clock, decreasing from the beginning of the interval to some point in the middle. The performance then became increasingly more accurate until the end of the interval. Under the chained schedules, accuracy also changed within the components. During the initial component, accuracy decreased from the beginning of the fixed interval to some point in the middle or at the end. During the middle component, the performance usually remained at an intermediate level of accuracy. During the terminal component, the initially inaccurate performance became increasingly more accurate throughout the interval. Systematic relationships between response rate and per cent error showed that all four pigeons performed most accurately at high rates. The accuracy of the performance at low rates was also quite high. These relationships held for all three types of schedules through an eight-fold variation in scheduled interreinforcement time.