The 5' untranslated regions (UTR) of chloroplast mRNAs often contain regulatory sequences that control RNA stability and/or translation. The petD chloroplast mRNA in Chlamydomonas reinhardtii has three such essential regulatory elements in its 362-nt long 5' UTR. To further analyze these elements, we compared 5' UTR sequences from four Chlamydomonas species (C. reinhardtii, C. incerta, C. moewusii and C. eugametos) and five independent strains of C. reinhardtii. Overall, these petD 5' UTRs have relatively low sequence conservation across these species. In contrast, sequences of the three regulatory elements and their relative positions appear partially conserved. Functionality of the 5' UTRs was tested in C. reinhardtii chloroplasts using beta-glucuronidase reporter genes, and the nearly identical C. incerta petD functioned for mRNA stability and translation in C. reinhardtii chloroplasts while the more divergent C. eugametos petD did not. This identified what may be key features in these elements. We conclude that these petD regulatory elements, and possibly the corresponding trans-acting factors, function via mechanisms highly specific and surprisingly sensitive to minor sequence changes. This provides a new and broader perspective of these important regulatory sequences that affect photosynthesis in these algae.

This paper describes the simulation of the precise control of speed in servo systems using the digltal phase-locked-loop (PLL) method and analog/hybrld computer techniques. The PLL for motor speed control is essentially a hybrid feedback control system in which a reference square-wave signal is used to command servo velocity and position. The system consists of a phase comparator, a lowpass filter, the motor under control, and a voltage-controlled-oscillator (VCO) in the feedback path. The mathematical model of the motor and the low-pass filter are simulated on the analog computer. The input reference is a frequency-modulated (FM) digital signal. An integrated circuit is used as the phase comparator which compares the perturbed VCO feedback frequency and the input signal.The method introduced here is simple to learn and practical to implement in undergraduate control courses.It introduces the student to the basic concepts of PLL control, without overwhelming him with its complex problems.

This paper describes the simulation of the precise control of speed in servo systems using the digltal phase-locked-loop (PLL) method and analog/hybrld computer techniques. The PLL for motor speed control is essentially a hybrid feedback control system in which a reference square-wave signal is used to command servo velocity and position. The system consists of a phase comparator, a lowpass filter, the motor under control, and a voltage-controlled-oscillator (VCO) in the feedback path. The mathematical model of the motor and the low-pass filter are simulated on the analog computer. The input reference is a frequency-modulated (FM) digital signal. An integrated circuit is used as the phase comparator which compares the perturbed VCO feedback frequency and the input signal.The method introduced here is simple to learn and practical to implement in undergraduate control courses.It introduces the student to the basic concepts of PLL control, without overwhelming him with its complex problems.