A DNA-dependent RNA polymerase has been solubilized and partially purified from washed chloroplasts prepared from maize seedlings. The purified enzyme was completely dependent on added DNA after purification by phospho-or DEAE-cellulose chromatography. On glycerol density gradients, the enzyme ran ahead of a marker with a sedimentation constant of 18 S, indicating a molecular weight of 500,000 or more. The instability of the highly purified enzyme made intensive study difficult, but the properties of the enzyme purified by plhosphocellulose chromatography are reported. Template specificity varied during purification but the activity was always higher with denatured than with native DNA and the preference for maize DNA over calf-thymus DNA increased during purification. The enzyme required magnesium for optimal activity, was inhibited by salt concentrations in excess of 0.1 M, and had a temperature optimum of 480C. The chloroplast enzyme differed from similar activities so far reported from maize or other sources, pat ticularly in the high salt concentrations needed to elute it from phosphocellulose. The, soluble, DNA-dependent enzyme was not inhibited by either a-amanitin or by rifaniycin-SV under the assay conditions used.It is essential to any understanding of the control of chloroplast DNA transcription and, thus, of the degree of organelle autonomy, that the properties and subunit structure of the DNA-dependent RNA. polymerase (EC 2.7.7.6)

The motor cortex and cerebellum are thought to be critical for learning and maintaining motor behaviors. Here we use transcranial direct current stimulation (tDCS) to test the role of the motor cortex and cerebellum in sensorimotor learning in speech. During productions of "head," "bed," and "dead," the first formant of the vowel sound was altered in real time toward the first formant of the vowel sound in "had," "bad," and "dad." Compensatory changes in first and second formant production were used as a measure of motor adaptation. tDCS to either the motor cortex or the cerebellum improved sensorimotor learning in speech compared with sham stimulation ( n = 20 in each group). However, in the case of cerebellar tDCS, production changes were restricted to the source of the acoustical error (i.e., the first formant). Motor cortex tDCS drove production changes that offset errors in the first formant, but unlike cerebellar tDCS, adaptive changes in the second formant also occurred. The results suggest that motor cortex and cerebellar tDCS have both shared and dissociable effects on motor adaptation. The study provides initial causal evidence in speech production that the motor cortex and the cerebellum support different aspects of sensorimotor learning. We propose that motor cortex tDCS drives sensorimotor learning toward previously learned patterns of movement, whereas cerebellar tDCS focuses sensorimotor learning on error correction.

Highlights d Sensorimotor learning was observed during the fluid production of variable sentences d Sensorimotor learning for sentences transferred robustly to the production of words d The brain predicts the sensory consequences of variable, sentence-level speech d Sensory prediction errors rapidly drive precise changes in fluid speech production

No abstract

No abstract